Preheat oven to 425 degrees. Line a large baking sheet with parchment paper or foil, and mist with cooking spray.
Combine the balsamic vinegar, olive oil and minced garlic into a medium-sized bowl, then add in the sweet potatoes, onions and mushrooms and toss until evenly coated.
Spread the sweet potatoes, onions and mushrooms onto the baking sheet. Place in the oven and roast for 15-20 minutes. They’ll be cooked but not crispy— that’ll happen on your pizza!
Add your chopped spinach to the bowl and rub with any leftover olive oil and balsamic. The spinach will overcook if you pre-bake it, so set it aside for now.