Add portobello mushrooms to a shallow baking dish or large freezer bag. Set aside.
In a small mixing bowl, whisk together balsamic vinegar, olive oil, cumin, black pepper, paprika, garlic, and steak sauce (optional). Taste and adjust seasonings as needed.
Add sauce to the mushrooms and use a pastry brush to brush on all sides. Marinate on one side for 5 minutes, then the other side for 5 minutes.
In the meantime, prepare chimichurri by adding parsley, garlic, shallot, red pepper flake, olive oil, lemon juice, salt, and pepper to a medium mixing bowl and whisking to combine. Taste and adjust flavor as needed, adding more lemon juice for acidity, salt for flavor, or red pepper flake for heat. Add avocado and toss to combine. Set aside.
Heat a grill or a large skillet over medium heat. Cook on each side for 2-3 minutes, or until caramelized and deep golden brown. Brush on any remaining marinade while cooking to infuse more flavor.
To serve, top portobello steaks with avocado chimichurri. This makes a great standalone meal, but would also pair well with my Pecan Apple Salad, Pear Walnut Salad, or Vegan Mashed Potatoes! They would also pair well with grilled asparagus and/or cooked quinoa, millet, or brown rice. Best when fresh.
*3 cloves of garlic yield ~1 1/2 Tbsp minced garlic. *Chimichurri sauce loosely adapted from Bon Appetit. *Mushroom marinade adapted from my Portobello Mushroom Burgers in our 31 Meals eCookbook! *For added protein, top with toasted pumpkin seeds or serve with cooked millet, quinoa, or brown rice! *Nutrition information is a rough estimate calculated with 4 portobello mushrooms, all the olive oil, and without optional steak sauce.