Add the sliced bell peppers, portobello mushrooms, and red onion to a large ziploc bag with the marinade. Zip to seal and shake until veggies are fully coated. Marinate at room temperature for 30-60 minutes, flipping the bag over halfway through.
Preheat the oven to 450° F and arrange the marinated veggies in a single layer on a baking sheet. You might want to split them between two baking sheets. Bake for 20-25 minutes, until mushrooms begin to brown.
Remove the veggies from the oven and preheat the broiler to high. Place the veggies back in the oven about 3 inches from the heating element for 4-5 minutes until charred to your liking.