Step 1

Finely chop 1 garlic clove and place in a 9x13-inch baking dish. Add 3 tablespoons balsamic vinegar, 2 tablespoons olive oil, 2 teaspoons low-sodium soy sauce or tamari, 1/2 teaspoon Dijon mustard, and 1/8 teaspoon black pepper and whisk to combine.

Step 2

Remove and discard the stems from 4 portobello mushrooms. Add to the baking dish and let marinate at room temperature until almost all of the marinade is absorbed, flipping halfway through, at least 10 and up to 30 minutes. Meanwhile, heat an outdoor grill for direct, medium-high heat (375º to 400ºF).

Step 3

Scrub the grill grates clean if needed. Oil the grill grates with a paper towel dipped in vegetable oil. Place the mushrooms stem-side up on the grill. Cover and grill until tender and lightly charred, 4 to 5 minutes per side. Meanwhile, pick the leaves from 1 fresh basil sprig and finely chop.

Step 4

Transfer the mushrooms to a platter. Sprinkle with the basil and red pepper flakes, if using.