In a large pot or Dutch oven, heat the olive oil over medium heat. Add the diced onion, garlic, carrots, and celery. Sauté for about 5 minutes until the vegetables are softened.
Stir in the thyme, rosemary, salt, and pepper. Add the chicken pieces and cook until they are no longer pink, about 5-7 minutes.
Pour in the chicken broth and bring to a simmer. Add the heavy cream and gnocchi, stirring to combine. Cook for an additional 5-7 minutes until the gnocchi are tender.
If you prefer a thicker sauce, mix the cornstarch with water in a small bowl to create a slurry, then stir it into the pot and cook for another 2-3 minutes until thickened.
Stir in the frozen peas and cheddar cheese until melted and well combined.
If using a pie crust, preheat the oven to 400°F. Transfer the mixture to a baking dish, cover with the pie crust, and cut slits for steam to escape. Bake for 20-25 minutes or until the crust is golden brown.
If not using a pie crust, serve the creamy chicken and gnocchi mixture directly from the pot.