While the oven heats, set a pan over medium heat and add 1 tablespoon of olive oil. When the pan is hot, add the onion, green pepper, and celery and cook for 5-6 minutes until softened. Allow the mixture to cool for approximately 5 minutes.
Combine the vegetables and all the remaining ingredients, except 1 cup of the cheddar cheese, in a mixing bowl until well combined. Reserve the extra cup of cheese for the topping.
Scrape the mixture into a baking dish and top with the remaining cheese.
Bake for 20 minutes until the mixture is bubbling. Remove from the oven and serve hot.
Notes
Store leftovers in an airtight container in the fridge for up to 3 days. Reheat in a 325℉ oven or in the microwave until warmed through.