Preheat oven to 450 degrees and bring a pot of salted water to a boil.
Boil pasta shells (12 oz) to al dente, strain, and toss with a drizzle of olive oil or butter to prevent from sticking while they cool.
While pasta cooks, fold together ricotta cheese (16 oz.) and chopped spinach (6 oz.) in a bowl, and set aside.
Meanwhile, fry ground Italian sausage (1 lb.) in a skillet on medium high heat, and halfway through, add minced garlic, onion, and bell pepper. Finish sautéing until sausage is crispy and vegetables are caramelized.
Add canned tomatoes, drained to skillet and stir to combine. Remove from heat and set aside.
Spread a small amount of the meat mixture over the bottom of a 3 quart, or 13x9 inch casserole dish to prevent sticking.
Use a spoon to scoop the ricotta and spinach mixture into each shell, one by one, and nestle side by side in a single layer in the casserole dish until it is completely full. They can be squished in there, but you might have some extra leftover shells.
Spoon the rest of the meat mixture evenly over the shells.
Sprinkle the mozzarella cheese over the top. If desired sprinkle breadcrumbs, fresh basil, crushed red pepper, or grated Parmesan (optional).
Bake at 450 degrees until golden and bubbly, about 10-12 minutes.