Bring a large pot of lightly salted water to a boil. Cook pasta shells in boiling water until cooked through but firm to the bite, 12 minutes. Drain.
Cook sausage in a large skillet over medium heat until completely browned, 5 to 7 minutes.
Melt Monterey Jack cheese in a saucepan over medium-low heat, 2 to 3 minutes. Stir spinach, tomato, onion, and bell pepper into the cheese; cook until hot, 2 to 3 minutes. Remove from heat and cool slightly.
Spoon sausage, Monterey Jack cheese mixture, and ricotta cheese into the pasta shells; arrange in a 9x13-inch baking dish. Pour tomato sauce over the stuffed shells.
Bake in preheated oven until the sauce is hot and bubbling, 20 to 25 minutes.