Preheat your oven to 375°F (190°C).

Wash and chop all the vegetables into bite‑sized pieces so they cook evenly in the casserole.

Bring a pot of salted water to a boil. Add the potatoes and carrots and cook for about 5 minutes until they begin to soften. Drain and set aside.

Heat a skillet over medium heat. Add a small drizzle of olive oil and cook the diced onion for about 3–4 minutes until soft.

Add the minced garlic and cook for another 30 seconds until fragrant.

In the same skillet, melt the butter over medium heat. Sprinkle in the flour and whisk constantly for about 1 minute to create a roux.

Slowly pour in the milk while whisking to prevent lumps. Then add the heavy cream.

Cook for 3–4 minutes until the sauce thickens slightly.

Season with salt, pepper, thyme, and onion powder.

Add the cooked potatoes, carrots, broccoli, and cauliflower into a large mixing bowl. Pour the creamy sauce over the vegetables and stir gently so everything is coated.

Transfer the mixture into a greased 9x13 baking dish.

Sprinkle shredded cheddar and mozzarella evenly across the top.

In a small bowl mix breadcrumbs with olive oil, then sprinkle over the cheese layer.

Place the baking dish in the oven and bake for 30–35 minutes until the casserole is bubbling and the top becomes golden brown.

Let the casserole rest for about 5 minutes before serving. This helps the sauce thicken and makes it easier to scoop.