Preheat oven to 350 degrees and butter 9 x 13 casserole pan with 1/2 tbsp. butter.
Rinse quinoa several times in cold water prior to cooking. See recipe notes.
Then add quinoa & 2 1/2 cups of water to medium saucepan. Bring to boil, reduce heat and cover. Cook 12-15 minutes till liquid absorbed. Set aside.
While quinoa is cooking steam broccoli florets on stove top about 7-8 minutes and set aside.
Meanwhile add 2 tbsp. butter & olive oil to skillet on stove top over medium heat. Add onion, garlic & cook 2-3 minutes.
Next add sliced mushrooms, salt & pepper and continue to cook about 5 more minutes.
Remove onion mushroom mixture from pan and set aside. Meanwhile in same pan add the remaining 2 tbsp. butter and corn starch to form roux.
Add greek yogurt, half & half, garlic powder, & dried mustard. Continue to cook about 5-6 minutes until thickened. Pour the mushroom & onion mixture back into pan.
Add half of shredded cheddar cheese and stir till melted about 1 more minute.
Start by spooning cooked quinoa on bottom layer of buttered casserole dish and spread out evenly.
Next layer steamed broccoli florets, followed by mushroom sauce & top with remaining cheddar cheese.
Bake in preheated oven 30 minutes.