Wash Brussels sprouts and slice off ends. Slice Brussels sprouts in half. The hard shell should fall off the sprouts easily when you slice them. Discard shells.
Heat oven to 425F.
Peel onion and slice into thin strips.
Wash and slice mushrooms.
Place Brussels sprouts and onion in a bowl. Add 3 Tablespoons of extra virgin olive oil, 1 Tablespoon of Balsamic vinegar, 1 minced garlic clove (or 1/8 tsp of garlic powder if using) and pepper to taste to bowl. Stir to combine ingredients. Place Brussels sprouts and onion in a large baking pan lined with parchment paper. Roast for 30 minutes or until Brussels sprouts are caramelized (lightly browned).
Place mushrooms in a bowl. Add 2 Tablespoon of extra virgin olive oil, 2 teaspoons of Balsamic vinegar , 2 minced garlic cloves (or 1/4 tsp or garlic powder if using) and pepper to taste to bowl. Stir to combine. Place mushrooms in a large baking pan lined with parchment paper. Roast in 425F oven for 18-20 minutes or until mushrooms are lightly browned.
Combine Brussels sprouts, onion and mushrooms in a casserole serving dish and serve immediately.
