Preheat the oven to 375°F (190°C).
In a medium saucepan, bring the beef broth to a boil. Add the rinsed quinoa, reduce heat to low, cover, and simmer for 15 minutes or until quinoa is tender and liquid is absorbed. Remove from heat and fluff with a fork.
While quinoa cooks, heat olive oil in a large skillet over medium heat. Add diced onion and cook for 3-4 minutes until softened.
Add minced garlic to the skillet and cook for 1 minute until fragrant.
Add ground beef to the skillet. Cook, breaking it apart with a spoon, until browned and cooked through, about 6-8 minutes. Drain excess fat if necessary.
Stir in diced bell pepper, diced zucchini, and drained diced tomatoes. Cook for 5 minutes until vegetables are slightly tender.
Add ground cumin, chili powder, salt, and black pepper. Stir well to combine.
Remove skillet from heat and stir in cooked quinoa until evenly mixed.
Transfer the mixture to a greased 9×13 inch casserole dish. Sprinkle shredded cheddar and mozzarella cheese evenly over the top.
Bake in the preheated oven for 15-20 minutes until cheese is melted and bubbly.
Remove from oven and let cool for 5 minutes before serving.