Preheat oven to 350 degrees.
Cook quinoa according to package. (I used an Instant Pot. Rinse the quinoa then add to the instant pot with 1 1/3 cups of water and set for 1 minute high pressure. Let it naturally release for 10 mins.)
Add carrot and potato pieces to a small pot then cover with water. Bring to a boil then cook for about 5 min, until fork tender.
Remove veggies from water with a slotted spoon and add to blender with all the other cheese sauce ingredients. Blend until smooth.
Heat butter or olive oil in oven-safe cast iron skillet (If you don't have an oven safe skillet, cook veggies first then add to a baking dish). Once hot, add onion and mushrooms and sauté for about 7 minutes, until the mushrooms are nicely browned. Turn off heat.
Make panko topping. In a small skillet heat 1 tablespoon of butter then add the panko and garlic powder. Stir constantly and cook for 2 minutes, until the breadcrumbs are golden brown.
Add the cooked quinoa, broccoli, and cheese sauce back to the skillet with the mushrooms. Mix well. Use a spatula to press the casserole flat down into pan. Sprinkle with the panko topping.
Bake for 20-25 min at 350. Let cool for 5 minutes, then enjoy!