Preheat your oven to 375°F (190°C).
In a skillet, heat olive oil over medium heat. Sauté onions until translucent, about 3–4 minutes.
Add minced garlic and cook for an additional minute, until fragrant.
Stir in the mixed vegetables and thyme, cooking for another 5 minutes until tender.
In a bowl, combine heavy cream, salt, and pepper, then pour over the sautéed mixture.
Place the fish fillets in a greased casserole dish, and pour the creamy vegetable mixture over them.
Sprinkle shredded cheddar cheese evenly on top, followed by breadcrumbs.
Bake in the preheated oven for 25–30 minutes, or until bubbly and golden brown.
Let the casserole rest for 5 minutes before serving.