I made this as a "guest chef" for my bosses party and blew EVERYONE out of the water!
It's intermediate level, so give yourself enough time to cook everything, stuff the shells, and have some wine ;)
In a large skillet, heat olive oil on med-high and saute onions till clear, then add sausage, salt-n-pep and crushed red pepper and 1 TBsp garlic. Cook till golden brown breaking up the pieces.
Remove from heat and transfer to large bowl to cool.
When mixture has cooled to room temp, stir in ricotta, spinach, 1/2 cup parm and sugar. Combine well.
Lightly grease a deep 9×13 casserole or lasagna pan and set aside.
Bring a large pot of salted water to boil and cook the pasta shells until just al dente. Drain and rinse under cold running water then pat dry.
Stuff the pasta shells with the sausage-spinach mixture and place in prepared casserole dish.
In a large saucepan, heat olive oil on medium high and saute remaining onions until clear. Add remaining garlic and cook until fragrant approx 1-2 minutes
Add the crushed tomatoes, salt-n-pep, crushed red pepper (t.t.) and cook for 5 minutes
Add the heavy cream and cook until sauce is reduced by 1/3, about 20 minutes
Preheat oven to 350F
Add the finely chopped basil to the sauce right before pouring the sauce over the shells in casserole.
Sprinkle with the fontina cheese and remaining Parm. Cover casserole tightly with aluminum foil and bake for 45 minutes Remove the foil and continue baking until the sauce is lightly browned in spots and bubbly around the edges. About 15 mins longer.
Let stand for 10 mins before serving. Goes great with thin sliced garlic bread.