Make Your Marinade and Preheat Your Oven: Preheat the oven to 400 F. In a large bowl, make the steak’s marinade while combining olive oil, balsamic vinegar, spices and chopped garlic.
Add the Marinade to the Portobello Steaks: Place portobellos and asparagus in an oven-safe skillet. With a pastry brush, generously brush the marinade on the tops and bottoms of both mushrooms and asparagus.