Preheat the oven to 375°F. Bring a large pot of salted water to a boil and cook the jumbo pasta shells until just al dente. Drain and set aside to cool slightly.

Heat olive oil in a large skillet over medium heat. Add the ground Italian sausage and cook until browned, breaking it up with a spoon as it cooks.

Add the diced onion and minced garlic to the skillet. Cook for 3 to 4 minutes until the onion softens. Stir in the chopped spinach and cook until wilted. Remove from heat and let cool for a few minutes.

In a mixing bowl, combine the ricotta cheese, egg, grated Parmesan, Italian seasoning, salt, and pepper. Add the cooled sausage and spinach mixture and stir until evenly mixed.

In a separate bowl, stir together the tomato sauce and heavy cream. Spread half of this sauce across the bottom of a 9x13 baking dish.

Spoon the filling into each cooked shell and arrange them in the baking dish. Pour the remaining tomato cream sauce over the top.

Sprinkle the shredded mozzarella cheese evenly over the shells. Cover with foil and bake for 20 minutes. Remove the foil and bake for another 10 minutes until the cheese is bubbly and lightly browned.