Preheat your oven to 350°F (180°C). Grease a 9x13-inch (23x33 cm) baking dish with olive oil or butter.
Heat olive oil in a large skillet over medium heat. Add the chopped onion, diced bell pepper and sliced mushrooms, and cook for 8-10 minutes, until the vegetables are tender and most of the moisture has evaporated.
In a large bowl, whisk together the eggs, cottage cheese, salt, pepper, and garlic powder until well combined.
To the egg mixture, add shredded cheese, freshly chopped spinach, and cooked vegetable mixture. Stir until evenly combined.
Pour the mixture into the prepared baking dish and spread evenly. Sprinkle Parmesan cheese over the top.
Bake for 25-30 minutes, or until the center is set and the top is lightly golden.
Let the casserole cool for at least 10–15 minutes before slicing. This helps it set and makes clean slices easier. Serve warm or at room temperature.