Preheat the oven to 200 degrees F; position the rack in the middle of the oven.
In a large pot, heat the oil over medium-high heat until it reaches 350 degrees F on a deep-frying thermometer. Line a rimmed baking sheet with aluminum foil.
In a large bowl, whisk together the flour, baking powder, salt and pepper. Set aside. In a separate large bowl, stir together the ricotta, mozzarella, Parmigiano-Reggiano, milk and eggs until combined. Add the flour mixture to the cheese mixture and mix until a thick and sticky batter forms. Cover and set aside at room temperature for 30 minutes.
Working in batches, with two large spoons, scoop approximately 2 tablespoons of the batter and carefully slide it into the hot oil. Don't overcrowd the pot. Cook the fritters until deep golden brown, 4 to 5 minutes, turning them in the oil from time to time. (To get nice, smooth fritters, periodically dip the spoons in the hot oil; it will allow you to get a "clean" scoop.) If the oil starts to gurgle and sputter, and the temperature has gone way up, you may need to reduce the heat to medium or even medium-low and gently bring it back up to 350 degrees F as needed.
Use a spider to remove the fritters from the oil and transfer to the prepared pan. Place the fritters in the oven to keep warm while you repeat with the remaining batter. Serve immediately, with Spicy Tomato Sauce.
Spicy Tomato Sauce:
Yield:About 6 cups
In a large. Wide saucepan, heat the oil over medium heat until shimmering. Add the onions and garlic and cook, stirring, until the onions are translucent, light golden, and beginning to brown, about 10 minutes. Add the tomatoes, basil, red pepper flakes, pepper, and salt to taste. Reduce the heat to low and simmer, uncovered, until the sauce has thickened, 35 to 45 minutes.
Use immediately or transfer to an airtight container and refrigerate or freeze until ready to use. The sauce will keep for up to 1 week in the refrigerator or up to 2 months in the freezer.
Yield:A generous 1 cup
In a medium nonreactive saucepan, combine the milk, cream, and salt and heat the mixture over medium heat, stirring from time to time to keep it from scorching on the bottom, until it registers 190 degrees F on a candy thermometer. Remove from the heat, add the lemon juice, and gently stir the mixture a couple of times. Let sit, undisturbed, for 5 minutes. The mixture will look curdled, with curds distinctly separating from the whey.
Meanwhile, line a colander with a few layers of cheesecloth and place it over a large bowl. Gently pour the curds and whey into the colander and let drain for at least 1 hour. After 1 hour, the ricotta will be soft and spreadable; after 2 hours, it will be somewhat firm but still spreadable. The ricotta will firm up further in the refrigerator. Discard the whey or save it for another use. Serve right away or transfer it to an airtight container and refrigerate until ready to use. The fresh ricotta will keep for up to 4 days.