Preheat oven to 400 degrees F (204 C) and line a large baking sheet with parchment paper. Set aside.
Add portobello mushrooms (stem side up) to a shallow dish or large freezer bag. Add the balsamic vinegar, oil, salt and pepper (or red pepper flake). Use a pastry brush to brush on all sides (or shake bag to coat). Marinate on one side for 5 minutes, then the other side for 5 minutes.
In the meantime, add vegetables and garlic to baking sheet, toss with oil, salt and pepper, and fresh herbs (optional) and bake for 20-25 minutes or until golden brown and fragrant, tossing once near the halfway point to ensure even baking. Set aside but keep oven at 400 F (204 C).
If serving with balsamic reduction (see notes) and/or soft nut cheese (see links above), prepare at this time.
Heat a large skillet over medium heat (you could also use a grill for this step). Add mushrooms to the pan, leaving any leftover oil-and-vinegar marinade behind. Cook on each side for about 4-5 minutes, or until caramelized and softened. If your mushrooms are extra thick, covering with a lid can be helpful to encourage even cooking all the way through. Brush on any remaining marinade while cooking to infuse more flavor.
Once mushrooms and vegetables are cooked, assemble pizzas. Place mushrooms on a parchment-lined baking sheet or baking pan and add toppings, starting with pizza sauce, then roasted veggies, roasted garlic, and soft vegan cheese. You can also add vegan parmesan cheese or red pepper flake at this point (optional).
Bake for about 15-20 minutes or until hot. The soft vegan cheese will be slightly golden brown on top. Remove from oven and enjoy as is, or garnish with sun-dried tomatoes, fresh basil, balsamic drizzle, red pepper flake, and extra vegan parmesan (all optional).
Best when fresh, though leftovers will keep covered in the refrigerator up to 3-4 days. Reheat in a 350 degree F (176 C) oven until hot.
*Recipe adapted from our Vegan Portobello Pizzas and inspired by the Stuffed Portobello Mushroom at ABGB. *For the red sauce, we adapted Smitten Kitchen’s favorite red sauce, straining one 28-ounce jar whole peeled San Marzano tomatoes for 2 hours and discarding the liquid that drained out. Strained tomatoes are blended with 3-4 Tbsp (1 small handful) basil, 1-2 cloves garlic, and salt to taste. *To make the balsamic reduction, add 1 cup (240 ml) balsamic vinegar to a small saucepan. Bring to a gentle boil. Allow to simmer until it is reduced by half and appears slightly syrupy (about 10-15 minutes). Watch carefully near the end as it can go from reduced to burnt quickly. You’ll know it’s done when it coats the sides of the pan when swirled. Remove from heat and let cool where it will continue to thicken. Store leftovers in the refrigerator for 2 weeks (sometimes longer). *Nutrition information is a rough estimate calculated with avocado oil and our macadamia ricotta recipe and without optional ingredients.