This satisfying dish can serve as breakfast, lunch, or dinner.
Heat oven to 425 degrees. Heat olive oil in a 10-inch nonstick skillet with an ovenproof handle over medium-high heat. Add sausage and cook, breaking up the meat, until well browned, about 2 to 3 minutes. Using a slotted spoon, transfer sausage to a bowl; set aside. Discard the fat and wipe out skillet.
Return skillet to medium heat; melt butter in it. Add eggs, sprinkle with salt, and cook until eggs begin to set on the bottom, about 10 seconds. Using a rubber spatula, pull edges into the center, tilting skillet so eggs fill the gaps. Sprinkle 1 cup mizuna or watercress, sausage, pecorino or Gruyere, and ricotta over eggs. Continue pulling the edges toward center. Cook until nearly set, about 4 minutes.
Transfer skillet to oven; bake until top is set and edges are golden brown, about 6 to 8 minutes. Sprinkle with pepper to taste and serve immediately or at room temperature with remaining mizuna or watercress and Spicy Tomato Sauce.