Place the heavy cream, crushed tomatoes, diced tomatoes with juices, minced garlic, Italian seasoning, salt, and pepper into the slow cooker and stir to combine. Cover and cook on low for 2 hours to let the sauce simmer and the flavors meld.

While the sauce cooks, bring a large pot of salted water to a boil. Cook the lasagna noodles according to package directions until al dente, then drain and rinse briefly with cool water to stop the cooking process. This prevents them from becoming mushy when stuffed.

In a large bowl, combine the ricotta cheese, half of the mozzarella, the Parmesan, the beaten egg, and the heavy cream. Mix until smooth and creamy.

Heat the olive oil in a large skillet over medium heat. Add the fresh spinach and sauté until just wilted, about 2 minutes. Remove from heat and let cool completely, then squeeze out any excess moisture using a clean kitchen towel or paper towels. This step is crucial to keep the filling from turning watery.

Fold the cooled, drained spinach into the ricotta mixture. Spoon about 2 tablespoons of the spinach-ricotta filling onto the center of each lasagna noodle, roll it up tightly like a cigar, and place seam-side down into the slow cooker, arranging them snugly in the tomato sauce.

Sprinkle the remaining mozzarella cheese over the top of the shells. Cover and cook on low for an additional 2 hours until the shells are tender and heated through. Do not stir after adding the shells to avoid breaking them.