Preheat oven to 375ºF and spray a casserole dish with olive oil.
Place quinoa, onion, and broccoli florets on the bottom of your casserole dish. Set aside.
Heat a saucepan over medium/high heat. Add olive oil and garlic. When the garlic is fragrant, add the milk, broth, and salt into a medium saucepan. Gently whisk until the liquid is hot.
Remove the pan from the heat and slowly add 6 oz. of grated cheese and whisk until the cheese dissolves into the mixture.
Pour liquid on top of the ingredients in a casserole dish and stir to combine.
Cover the casserole dish and place it in the oven at 375ºF for 30 minutes. Remove and stir.
Bake for an additional 20 minutes, uncovered.
While baking, add all of the ingredients for the topping to a bowl and mix until a crumble forms.
Spread the topping over the top of the casserole and broil until the topping turns golden brown.
Remove from the oven and let it rest for 5 minutes before serving.