Rinse quinoa under cold water, then combine with vegetable broth or water in a saucepan. Bring to a boil, reduce to low, cover, and simmer for 15 minutes.

Heat olive oil in a skillet over medium heat. Sauté the chopped onion for about 5 minutes until soft, then add garlic and cook for 1 minute.

Add chopped broccoli florets and a splash of water to the skillet. Cover and steam for 3 to 5 minutes until tender-crisp.

In a small bowl, whisk together milk, Greek yogurt (or sour cream), Dijon mustard, and salt and pepper until smooth.

Stir the cooked quinoa into the skillet, then add the sauce and mix thoroughly. Fold in cheddar and Monterey Jack cheeses.

Preheat oven to 375°F (190°C). Grease an 8x8-inch baking dish and transfer the mixture, spreading it evenly. Optionally, sprinkle breadcrumbs on top.

Bake for 15 to 20 minutes until bubbly and golden brown. Allow to rest for 5 minutes before serving.