Smoky, savory portobello steaks marinated in balsamic, tamari, garlic, and a hint of liquid smoke, then seared until meaty, tender, and deeply umami.
In a large, shallow baking dish, whisk together all marinade ingredients and taste to adjust seasoning.
Add the portobello mushrooms and liberally brush or spoon marinade over all sides. Marinate 10–20 minutes, flipping and brushing again halfway through.
Heat a grill, grill pan, or skillet over medium-high heat until hot. Brush the cooking surface with oil.
Place mushrooms on the hot surface (they should sizzle). Do not move them; press gently with a spatula or tongs and sear 4–5 minutes.
Flip, add a little more oil if needed, and sear the second side 4–5 minutes, brushing with additional marinade if desired, until browned and tender.
Serve immediately and enjoy.