Preheat your oven to 400°F (200°C).
Cut the chicken breast into 1-inch bite-sized cubes.
Chop the bell pepper, slice the zucchini into half-moons, and slice the mushrooms.
In a large mixing bowl, toss the chicken and vegetables with olive oil, dried oregano, garlic powder, sea salt, and black pepper until evenly coated.
Transfer the chicken and vegetable mixture into a medium-sized baking dish.
Pour the marinara sauce over the mixture and stir gently to ensure everything is well-incorporated.
Bake for 20 minutes until the chicken is cooked through and the vegetables are tender.
Sprinkle the shredded mozzarella cheese over the top and return to the oven for 3-5 minutes, or until the cheese is melted and bubbly.
Remove from the oven and let the bowl rest for 2 minutes before serving hot.