Preheat your oven to 375°F (190°C). Grease a 9x13-inch casserole dish.

Rinse the quinoa thoroughly under cold water. Combine quinoa and vegetable broth in a medium saucepan. Bring to a boil, then reduce heat and simmer covered for 15 minutes until liquid absorbs. Fluff with a fork and set aside.

While quinoa cooks, heat olive oil in a large skillet over medium heat. Add diced onion and sauté for 3-4 minutes until softened.

Add minced garlic to the skillet. Cook for 30 seconds, then add bell peppers and zucchini. Sauté for 5-6 minutes until vegetables soften slightly.

Stir in fresh spinach and cherry tomatoes. Cook for 2 minutes until spinach wilts. Remove from heat and season with Italian seasoning, salt, black pepper, and red pepper flakes if using.

In a large mixing bowl, combine cooked quinoa and sautéed vegetables. Mix thoroughly. Add beaten eggs and 1½ cups of shredded cheese. Stir until everything combines evenly.

Transfer the mixture to your prepared casserole dish. Spread evenly and press down gently. Sprinkle remaining cheese on top.

Bake uncovered for 30-35 minutes until the cheese melts and turns golden brown.

Remove from oven and let rest for 5 minutes before serving.