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, ½ of the diced
mozzarella, a couple of handfuls of Parmigiano and
½ of the Asiago.
To a small saucepot over moderate heat
add oil, garlic and onions.
Sauté onions and garlic 5 minutes.
Add tomatoes and season sauce with
salt and pepper.
Simmer sauce 5 minutes and stir in
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
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.