Make the marinara: Heat oil in large saucepan over medium heat. Add garlic, saute for 1 minute (do not burn!). Stir in tomatoes, vinegar, Italian seasoning. Add salt, pepper and sugar to taste (listed amounts are what I use). Reduce heat and simmer for 1/2 hour (or more). If you want the sauce smoother, put in blender to give that consistency.
Cook pasta according to package directions. Drain, rinse in cool water and set aside.
Preheat oven to 375 degrees. Coat 2 13"x9" baking dishes with olive oil or cooking spray, set aside.
Combine ricotta cheese, Asiago cheese, Parmesan cheese, egg, parsley, salt, pepper and spinach in a bowl.
Spoon approximately 1 tablespoon of the cheese mixture into each shell (or use heavy-duty zip-top bag as a pastry bag and pipe into the shells). Arrange half of the shells in each dish with seam sides up. Pour half of marinara over shells in one dish, other half of marinara over shells in the other dish. Top each dish with 1/2 cup of mozzarella cheese.
At this point, you can freeze one casserole for later use: cover with plastic wrap, pressing to remove as much air as possible. Wrap with heavy-duty foil. Store in freezer up to 2 months. Then, bake frozen casserole in preheated 375 degree oven for 1 hour and 10 minutes. MAKE SURE YOU REMOVE THE PLASTIC WRAP FIRST, then recover with foil for baking!
Cover the other casserole with foil. Bake at 375 degrees for 30 minutes or until thoroughly heated. Garnish with additional Parmesan cheese, if desired.