Fold the spinach leaves in half and strip off their stems (unless you’re using young tender spinach). Wash the leaves in several changes of water until totally free of grit; drain in a colander. Put the wet leaves in a large saucepan, cover, and cook over medium heat until the spinach is wilted, tossing the leaves once or twice, 5 to 7 minutes. Drain and let cool. Squeeze handfuls of spinach to extract as much water as possible, and then chop it finely.
Heat 1 Tbs. butter in a medium sauté pan and sauté the chopped onion until soft, 3 to 4 minutes. Add the chopped spinach, along with the garlic, nutmeg, salt, and pepper. Sauté, stirring often, until the spinach is quite dry, 3 to 4 minutes. Taste and adjust seasoning. Set aside.
Heat the oven to 400°F. Butter six 1-cup ramekins twice. Bring the milk just to a boil in a small pan. Melt 4 Tbs. butter in a medium pan; stir in the flour. Cook this roux over medium until it foams, 1 minute. Whisking briskly, pour in the hot milk; keep whisking as the sauce boils and thickens, 1 to 2 minutes. Reduce the heat; simmer 1 to 2 minutes. Season with a small pinch of nutmeg, and salt and pepper to taste. Pour about a third of the sauce back into the small pan and pour the cream on top to keep a skin from forming; set aside.
Stir the spinach into the larger portion of the white sauce; warm it slightly. It should be soft enough to fall easily from a spoon without being soupy. Taste and adjust the seasonings; the mixture should be highly seasoned to balance the bland egg whites that will be added. Take the pan from the heat. Stir the yolks in one by one; the mixture should thicken slightly. If necessary, reheat 1 to 2 minutes, stirring, to thicken. Keep the pan warm.
Start whisking the egg whites slowly, whether you’re using a mixer or a whisk. Gradually increase to full speed, lifting the whisk high up from the bowl if beating by hand. Beat until the whites form soft peaks, 2 to 4 minutes. Finally, if working by hand, “tighten” the whites by whisking in large circles down in the bottom of the bowl for about 30 seconds.
The whites should be firm enough to hold a definite peak when the whisk is lifted, but the peaks shouldn’t be stiff or choppy. The whites will look smooth and matte, and they’ll cling to the bowl with no trace of granular “curdling."
Check that the spinach mixture is very warm; if necessary, reheat it, stirring constantly. Add about a quarter of the whipped egg whites to the warm spinach and stir them together, scooping to the bottom of the pan to mix them thoroughly. The heat will cook the egg whites slightly and stiffen the mixture.
Tip this mixture into the remaining whites and, with a metal spoon or a rubber spatula, cut down into the center of the bowl, scoop under the contents, and turn them over in a rolling motion. At the same time, turn the bowl in the opposite direction. Keep folding gently just until the mixture is smooth. If it starts to lose volume and get sloppy, stop—a few bits of unmixed egg white are better than a flat soufflé.
Gently spoon the soufflé mixture into the buttered ramekins. Run a metal spatula or knife across the top so the mixture is smooth and level with the rim, letting the excess fall back into the bowl. Run your thumb around each dish just inside the rim to make a groove so the soufflé will rise in a straight “hat.”
Set the ramekins in a roasting pan. Pour very hot water around them to come two-thirds of the way up the sides, taking care not to let any splash onto the souffle mixture. Put the pan in the heated oven and bake the soufflés until they puff well above the rim of the dish and brown lightly on top, 20 to 25 minutes. Press the center of the tops with a fingertip—they should be just firm. Remove them from the oven, let them cool 4 to 5 minutes, and then lift out the ramekins to cool completely on the counter. As they cool, they’ll shrink.
Lightly butter the baking dishes. When the soufflés are cool, run a knife around the edge of each dish, pull the mixture from the sides with your fingers and let the soufflé fall upside down into the buttered baking dishes.
Whisk the reserved white sauce with the cream, bring it to a boil, and taste it for seasoning (the sauce should be very light, barely thickened). Spoon this sauce over the soufflés, completely coating both them and the bottom of the dishes. Sprinkle the tops with the shredded Gruyère.
Heat the oven to 425°F and position the bottom shelf quite low. Set the soufflés on a baking sheet. Bake the soufflés until they’re puffed and browned and the sauce is bubbling, about 10 minutes (if your dishes are very cold, it may take longer). Serve them immediately.
You can bake the soufflés, transfer them to their baking dishes, cover with white sauce and cheese, and then cover with plastic wrap and refrigerate for up to 24 hours before baking a second time. When you're ready to serve, take the soufflés from the fridge and put them in the heated oven without letting them come to room temperature. The second baking will be a few minutes longer.