Caramelize the onions: In a skillet, melt 3 tablespoons of unsalted butter over medium heat. Add 2 large sweet onions and cook, stirring frequently, until soft and golden brown, about 15-20 minutes.

Season the onions: Once caramelized, add kosher salt, black pepper, 1 teaspoon dried thyme, and 2 minced cloves of garlic. Stir and cook for an additional 2-3 minutes.

Mix the filling: In a bowl, combine 15 ounces of ricotta cheese, 1 beaten egg, 1 cup of grated gruyere, 1/2 cup of grated parmesan, and 1 tablespoon of flour. Stir until well incorporated.

Fill the shells: Gently take 12 jumbo shells and stuff with the ricotta mixture, ensuring they are packed but not bursting.

Prepare the sauce: In a baking dish, combine 2 cups of chicken stock, 1/2 cup of cream, and season with additional kosher salt and black pepper. Mix well.

Arrange the stuffed shells: Place shells in the baking dish on top of the sauce. Sprinkle 1 cup of mozzarella and 1/2 cup of seasoned breadcrumbs over the top.

Bake: Preheat oven to 350°F (175°C) and bake for about 25 minutes, until cheese is bubbly and golden.

Optional: Garnish with fresh parsley or thyme before serving.