How to Make It
Add the vegetable base, mustard, vinegar, garlic, sugar, basil and black pepper to a small mixing bowl. Slowly whisk in the vegetable oil. Add the marinade to a re-sealable plastic bag and add the portabella mushrooms. Place the mushrooms in the refrigerator for 1 1/2 hours and up to 8 hours.
Preheat the grill to 400°F.
Remove the mushrooms from the marinade and place directly onto the grill. Grill for 2 - 3 minutes per side. Remove the mushrooms from the grill and serve immediately as a side dish or dress as a burger.