Preheat the oven to 350 degrees F (175 degrees C). Grease an 8-inch square baking dish or pie pan.
Heat butter and olive oil in a skillet over medium-high heat; add onion. Cook and stir until softened, about 10 minutes; transfer onion to a plate to cool.
Whisk cream, eggs, salt, and black pepper in a bowl; stir in mozzarella cheese, Parmesan cheese, and onion. Pour mixture into the prepared baking dish.
Bake in the preheated oven until eggs are set and a knife inserted in the center comes out clean, about 35 minutes. It should be light brown around the edges but not brown on top.
Nutritional Facts
Per 8 servings
Calories: 168
Carbohydrate: 2g
Fat: 15g
Fiber: 0g
Protein: 7g
Sugar: 1g
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