Whisk together the balsamic vinegar, olive oil, soy sauce, minced rosemary, garlic powder, black pepper, cayenne, and smoked paprika in a shallow dish to create a well-balanced marinade with sweet, savory, and spiced notes. The vinegar provides acidity and depth while the soy sauce adds umami richness. Place the cleaned portobello mushroom caps gill-side up in the marinade and coat both sides thoroughly. Let them sit for 5 minutes per side, allowing the flavors to penetrate the mushroom flesh. I find that marinating the mushrooms helps them develop a deeper, more complex flavor compared to seasoning them dry.

While the mushrooms marinate, preheat your grill or grill pan to medium-high heat (around 375-400°F). Once heated, lightly brush or rub the grill grates with canola oil to prevent sticking and create better char marks. Remove the mushrooms from the marinade with a slotted spoon or tongs, allowing excess liquid to drip back into the dish. Reserve the remaining marinade in the dish—you'll use this as a basting liquid while cooking to build layers of flavor.

Place the mushrooms gill-side down on the hot grill and cook for 3-4 minutes without moving them, allowing them to develop a golden crust and char marks. Flip the mushrooms carefully and cook the other side for another 3-4 minutes. As they cook, brush the reserved marinade onto the mushroom caps using a pastry brush, which caramelizes the surface and intensifies the flavor. I like to baste them multiple times during cooking—this creates a glossy, flavorful exterior while keeping the interior tender. The mushrooms are done when they're tender when pierced with a fork and have developed nice grill marks.

Transfer the grilled mushrooms to a serving platter while they're still warm. Garnish generously with fresh chopped parsley to add brightness and color to the finished dish. Serve immediately while the mushrooms are at their peak texture—tender inside with a flavorful caramelized exterior.