Preheat your oven to 375°F (190°C).
In a skillet, heat olive oil over medium heat and sauté the onion until softened.
Add garlic and cook until fragrant.
Mix in shredded chicken, chicken broth, nutmeg, thyme, salt, and pepper. Simmer for 5 minutes.
Spread the chicken mixture in a casserole dish and layer the mashed potatoes evenly on top.
Sprinkle Gruyère cheese and breadcrumbs over the mashed potatoes.
Bake for 20–25 minutes, or until golden and bubbling.
Serve warm and enjoy.