Preheat the oven to broil with a rack in the upper third. Melt the butter in a 10-inch nonstick ovenproof skillet over medium-high. Add the sausage and break up meat into small pieces with a wooden spoon then spread out in the pan to create a single layer. Cook, undisturbed, until the bottom side is browned and crisp, about 3 minutes. Break up meat again and turn over pieces. Cook until the other side is browned and crisp and meat is cooked through, about 3 more minutes.
Increase the heat to high and add the spinach. Cook, tossing with tongs, until the spinach is wilted and all the liquid has evaporated, about 3 minutes.
Meanwhile, using a fork, beat the eggs with the milk until well combined. Beat in ½ cup of parmesan, 1/2 teaspoon salt, and ¼ teaspoon pepper.
Pour egg mixture into skillet; cook over medium-high, slowly stirring with a spatula, scraping the bottoms and sides, to create large curds, until eggs are wet on top but otherwise set throughout, 2 to 3 minutes.
Transfer to oven; broil until puffed and browning in spots and eggs are just set, about 3 minutes.
Remove from the oven and spread Rao’s Homemade® Arrabbiata sauce over the top of the cooked frittata. Arrange the mozzarella over the sauce. Return to the oven and broil until sauce is hot and cheese is golden and bubbly, 4 to 5 minutes.
Top with remaining 2 tablespoons of parmesan and basil leaves. Let stand 5 minutes before slicing.