Preheat the oven: Preheat your oven to 350°F (175°C).

Grease a 9x13 inch baking dish with cooking spray or olive oil.

Prepare the veggies by taking a a large skillet and heating the olive oil over medium heat.

Add the diced bell pepper, onion, mushrooms, and minced garlic.

Sauté for about 5 minutes until the vegetables are softened.

Add the chopped spinach and cook for an additional 2 minutes until wilted.

Season with salt and pepper to taste.

Whisk the eggs by taking a large mixing bowl and whisking together the eggs and milk until well combined.

Season with salt and pepper to taste.

Assemble the casserole by spreading the sautéed vegetables evenly in the greased baking dish. Scatter the halved cherry tomatoes on top.

Sprinkle the shredded cheese evenly over the vegetables.

Now, carefully pour the whisked eggs and milk mixture over the vegetables and cheese in the baking dish, ensuring it's evenly distributed.

Now place the baking dish in the preheated oven and bake for 25-30 minutes, or until the eggs are set and the top is golden brown.

Once baked, remove the casserole from the oven and let it cool slightly so it’s ready to be served.

Slice into squares or rectangles and serve warm.