Heat the oven to 400ºF. Butter a shallow 5- or 6-cup ceramic gratin dish. Melt 1 Tbs. of the butter and toss it with the breadcrumbs; set aside.
In a medium saucepan, bring the cream and garlic to a boil (watch that it doesn't boil over), immediately lower the heat, and simmer vigorously for 5 minutes; the cream should be reduced to about 3/4 cup. Take the pan off the heat and remove the garlic cloves with a slotted spoon. Let the cream cool slightly, stirring occasionally to loosen. Season it with a few grinds of fresh pepper and 1/4 tsp. of the salt.
Meanwhile, in a large (12-inch) nonstick skillet, cook the bacon over medium heat until crisp and browned. With tongs, transfer it to paper towels; crumble when cool. Leave the bacon fat in pan (if there's more than 2 Tbs., drain a little off). Add the remaining 1 Tbs. butter to the skillet and let it melt. Add the chard stems and sauté them over medium to medium-high heat until they're somewhat softened and browned on the edges, about 10 minutes. Reduce the heat to medium, add the chard leaves and toss them with the contents of the skillet. Season them with the remaining 1/4 tsp. salt. (You can add the leaves in two batches for easier handling.) Sauté until all the leaves are wilted, about 2 minutes. Use tongs to transfer the contents of the pan to the gratin dish (leave behind any excess liquid in the sauté pan), spreading them evenly.
Sauté sliced chard stems first until tender; then add the leaves and wilt them.
Sprinkle the crumbled bacon and then the cheese over the chard. Pour the seasoned cream over all and top with the buttered breadcrumbs. Bake for 25 minutes; the gratin will be brown and bubbly. Let rest for 10 to 15 minutes before serving.