Preheat oven to 425 degrees. In a baking dish, add the potato cubes, avocado oil and garlic and herb seasoning. Mix together in baking dish. (That way you don't need to clean an extra bowl too!)Add baking dish to oven and bake for 25 minutes.
While the potatoes are baking, cut chicken into 2" cubes. After 25 minutes, remove the baking dish from oven and add the chicken pieces and mushrooms.
Nestle the chicken pieces down into the potato mixture a bit. They should not be sitting on top. Add the onions. Add 1 more tbsp olive oil and more garlic and herb seasoning. Bake for 20-25 more minutes. Once done, check the chicken is fully cooked and the potatoes are soft and browned.
Remove from oven, add mozzarella slices on top. Top with a sprinkle of breadcrumbs. Add back to oven for 5 minutes until cheese is melted or browned.