Cook bacon in a nonstick skillet over medium-high heat until crisp. Using slotted spoon, transfer bacon to paper towels to drain. Pour bacon drippings into a bowl; reserve. Return 2 tablespoons drippings to skillet. Add shallots and sauté over medium heat until golden, about 4 minutes. Add greens and toss; sauté until wilted, tender, and dry, about 15 minutes. Transfer greens to plate; cool.
Preheat oven to 350°F.
Beat eggs and salt to blend in a large bowl. Whisk in 1/3 cup Parmesan, then greens and half of bacon. Stir in ricotta, leaving some clumps. Heat 1 tablespoon reserved drippings in an 8-inch ovenproof skillet or baking pan over medium heat. Pour in egg mixture; spread greens evenly. Sprinkle remaining bacon and Parmesan over eggs. Cook over medium heat until the frittata is just set at edges, about 10 minutes. Transfer to oven and bake until just set, about 20 minutes. Cut around frittata to loosen; slide out onto platter. Let cool 30 minutes. Slice into wedges and serve. Sprinkle with more Parmesan cheese if desired.