6 ounces Benton’s or slab bacon, cut into 1/2-inch pieces
4 tablespoons butter
2 medium yellow onions, peeled and thinly sliced lengthwise
1/2 teaspoon ground nutmeg
Sea salt and freshly ground black pepper, to taste
1-1/2 cups flour
1/2 teaspoon dry ground rosemary
1-1/4 cups milk
3 eggs, lightly beaten
1 cup Swiss cheese, grated
Heat bacon in a 12″ skillet over medium-high heat, and cook, stirring occasionally, until fat renders but bacon is not yet really crisp, about 8 - 10 minutes. Using a slotted spoon, transfer bacon to paper towels to drain; pour bacon fat into a 9″ × 11″ baking dish or jelly roll pan and set aside. Add butter, onions, salt, and pepper to the same skillet. Fry onions over medium-high heat, stirring frequently, until cooked through and just caramelized, about 15 minutes, add nutmeg and stir; remove from heat; set aside.
Heat oven to 425 degrees. In a large bowl, whisk together flour, salt and pepper; add milk and eggs, and stir until smooth. Let batter rest for 10 minutes. Meanwhile, place baking dish with bacon fat in oven and let heat for 10 minutes. Remove baking dish from oven, pour in batter, sprinkle with cheese and rosemary; and then the rendered bacon and caramelized onions; return to oven and bake until puffed and golden brown, about 30 minutes.