Position a rack in the center of the oven and heat the oven to 375°F.
Whisk the eggs and milk in a medium bowl. Add the spinach, half of the Parmigiano, 1/2 tsp. salt, and 1/2 tsp. pepper and mix well.
Melt the butter in a 10-inch oven-safe nonstick skillet over medium-low heat. Add the onion and cook, stirring occasionally, until translucent, about 3 minutes. Add the egg mixture and cook until the eggs begin to set, occasionally lifting the sides with a spatula to let uncooked egg run underneath, 3 to 5 minutes. Lay the tomato slices over the eggs and dollop the ricotta around the tomatoes. Sprinkle with 1/4 tsp. salt and a few grinds of pepper and then the remaining 1/4 cup of the Parmigiano. Bake until the ricotta is heated through, about 10 minutes. Let cool slightly before slicing and serving.
nutrition information (per serving):
13, Fat Calories
120, Saturated Fat
12, Monounsaturated Fat
4, Polyunsaturated Fat
Photo: Scott Phillips