Preheat the oven to 350 degrees F (175 degrees C).

Melt 1 tablespoon butter over medium heat in a large pot. Add bacon, onion, and garlic; cook until onion is soft, about 5 minutes. Add chicken; cover pot. Cook until chicken is no longer pink in centers, stirring occasionally, about 10 minutes.

Stir in broccoli, cauliflower, mushrooms, leek, chicken stock, and garlic and herb seasoning. Cover pot; cook until vegetables are soft, about 10 minutes.

Meanwhile, heat a saucepan over medium heat. Add heavy cream and remaining 2 tablespoons butter; cook until heated through, 2 to 3 minutes. Add cream cheese; cook until melted, stirring often, 3 to 4 minutes. Off heat, stir in Parmesan cheese.

Drain and discard excess liquid from chicken mixture, if necessary. Transfer chicken mixture to a large baking dish. Pour sauce on top; sprinkle with Cheddar cheese.

Bake in the preheated oven until cheese is melted and browned to your liking, about 10 minutes.