Melt 3 tablespoons of unsalted butter in a large skillet over medium heat. Add the sliced sweet onions and cook until golden brown, about 15-20 minutes.

Season the onions with kosher salt, black pepper, dried thyme, and minced garlic. Cook for another 2-3 minutes.

In a bowl, mix together 15 ounces of ricotta cheese, 1 beaten egg, 1 cup of gruyere cheese, 1/2 cup of parmesan cheese, and 1 tablespoon of flour until creamy.

Cook 12 jumbo shells according to the package instructions. Drain and let cool slightly before filling with the ricotta mixture.

In a 9x13 inch baking dish, pour in 2 cups of chicken stock and 1/2 cup of cream, seasoning with salt and pepper.

Place the stuffed shells in the baking dish and top with 1 cup of shredded mozzarella cheese and 1/2 cup of seasoned breadcrumbs.

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

Garnish with freshly chopped parsley or thyme before serving.