Prep the oven: Preheat your oven to 375 degrees Fahrenheit and lightly grease a 9x13 inch baking dish.
Cook aromatics: Heat olive oil in a skillet over medium heat. Sauté the diced onion and minced garlic for 5 minutes until soft.
Mix the base: Transfer the cooked onion and garlic to the prepared baking dish. Add the rinsed quinoa, dried lentils, vegetable broth, crushed tomatoes, oregano, cumin, salt, and pepper. Stir well to combine.
Cover and bake: Cover the baking dish tightly with aluminum foil. Bake for 45 minutes.
Add greens: Remove the foil carefully. Stir the fresh spinach into the hot mixture until the leaves wilt. Bake uncovered for an additional 10 minutes until the liquid is absorbed.