Preheat the oven to 350 ºF.
Butter/grease the inside of a deep 12 inch (30.5 cm) by 10 inch (25.5 cm) baking dish and set aside.
In a medium bowl, beat the eggs with a whisk.
Add the cheese and buttermilk to the eggs, and whisk until well combined. Set aside.
Melt the butter in a large pan on medium heat.
Add the onions and garlic and cook for about three minutes or until the onions starts to soften.
Add red peppers, mushrooms, salt and pepper and cook for about 7 minutes, stirring occasionally.
Add the broccoli and cook for another 2 minutes.
Pour the sautéed mixture into the baking dish.
Pour the egg mixture over the sautéed mixture.
Give the ingredients a quick stir with a spoon and place the baking dish on the middle rack of the hot oven.
Bake for 40 to 45 minutes or until the frittata looks set and the edges start to brown.
Serve hot or cold, on its own or with a favourite side. Enjoy!