In a shallow baking dish, whisk together the balsamic vinegar, olive oil, soy sauce, minced garlic, onion powder, paprika, cumin, smoked paprika, black pepper, and cayenne until well combined.

Add the cleaned portobello mushrooms to the dish and use a pastry brush to coat them on all sides with the marinade. Let them sit for at least 10 to 15 minutes, flipping once halfway through; longer marinating yields deeper flavor.

Meanwhile, preheat a grill or a large cast-iron skillet over medium-high heat. Lightly oil the grates or spray the pan with cooking spray.

Place the mushrooms on the hot grill or skillet. Cook for 4 to 5 minutes per side, brushing with any remaining marinade as they cook, until the mushrooms are tender and have deep golden-brown grill marks.

Remove from heat and let rest for a minute before serving. Enjoy them as a hearty main with your favorite sides, like a crisp green salad or roasted potatoes.