Preheat oven or broiler to 450 degrees.

Bring a large pot of water to a boil. Salt water and add pasta. Cook shells 12 to 15 minutes, they should be softened but still undercooked at the center. Drain pasta and cool.

Combine ricotta, 1/2 of the diced mozzarella, a couple of handfuls of Parmigiano and 1/2 of the Asiago. Add parsley to the cheeses and stir to combine.

To a small saucepot over moderate heat add oil, garlic and onions. Saute onions and garlic 5 minutes. Add tomatoes and season sauce with salt and pepper. Simmer sauce 5 minutes and stir in basil leaves

Pour a little sauce into the bottom of a shallow medium-sized casserole dish. Fill shells with rounded spoonfuls of cheese mixture and arrange them seam-side down in casserole dish. Top shells with remaining sauce and remaining mozzarella and Asiago cheeses. Place shells in very hot oven or 8 inches from hot broiler and cook 6 to 8 minutes to melt cheeses and bubble sauce.

This recipe yields 4 servings.