Prep and cook the greens:
To get the 2 cups of cooked greens you need for the gratin, be sure to start out with the amount of raw greens specified below.
Cut off and discard the tough stems (use a small, sharp paring knife and trim around the stem). Bring a large pot of lightly salted water to a boil, submerge all the greens, and cook just until tender (see cooking times below). Drain well and then spread on a towel to absorb excess moisture. If the greens still seem very wet, squeeze them gently to remove excess liquid.
Start with 1 lb. mature spinach, stems removed and leaves roughly chopped to yield about 7 cups tightly packed (or 12 oz.); cook for 30 seconds, just until wilted.
Start with a 1-lb. bunch broccoli raab, tough lower stems removed (almost half the bunch), as well as any discolored leaves, and the rest very roughly chopped to yield about 6 heaping cups; cook for 2 min.
Start with 1-3/4 lb. chard, stems cut away and reserved for another use (slice, freeze, and add to your next vegetable soup) and leaves roughly chopped to yield about 9-1/2 cups (or 12 oz.); cook for 1 min.
Start with 1-1/4 pounds kale, tough stems trimmed away, leaves roughly chopped to yield 6 cups tightly packed; cook for 8 min.
Assemble and bake the gratin:
Heat the oven to 400°F. Have ready a shallow 4-cup ceramic gratin dish or casserole dish (any shape is fine as long as it’s shallow). Melt 1 Tbs. of the butter and toss it in a small bowl with the breadcrumbs and a pinch of kosher salt and a little ground pepper; set aside.
In a medium saucepan, bring the cream and garlic to a boil over medium-high heat (watch that it doesn’t boil over), immediately lower the heat, and simmer vigorously until the cream reduces to about 3/4 cup, 4 to 8 min. (Don’t over-reduce.) Take the pan off the heat and remove and discard the garlic cloves. Let the cream cool slightly, stirring occasionally to keep a skin from forming. Season with 1/4 tsp. of salt and a few grinds of fresh pepper.
Meanwhile, in a large nonstick skillet, cook the bacon or pancetta over medium heat until crisped and browned, about 7 minutes. Transfer to paper towels, and carefully pour off most of the excess fat in the skillet (but don’t wipe it clean). Return the skillet to medium heat. Add the remaining 1 Tbs. butter to the skillet and let it melt. Add the cooked greens, season with 1/4 tsp. salt if using bacon (omit the salt if using pancetta), and cook, stirring constantly, for 1 min. Transfer the greens to the gratin dish and spread them evenly.
Crumble the bacon or pancetta over the greens. Sprinkle on the cheese. Pour the seasoned cream over all, and top with the buttered breadcrumbs. Bake until the gratin is brown and bubbly, about 25 min. Let rest for 10 to 15 min. before serving.
For a change, serve these gratins individually. Just divide ingredients among four small gratin dishes and bake as directed above.