Steps
1. Preheat oven to 350°F.
2. Heat butter in a skillet over medium heat. Add the mushrooms and garlic, stir frequently until the vegetables are tender.
3. Add in the diced onion until transparent.
4. Reduce heat and add in the flour, stir until well combined.
5. Whisk in the milk until the mixture is smooth. Simmer for 2 minutes.
6. Remove from heat and stir in the cheese and diced tomatoes.
7. Transfer the mixture to a greased baking dish.
8. Bake for 25-30 minutes or until cheese is melted and bubbly.
9. Serve with crusty bread or your favorite side dish. Enjoy!