These portobello mushroom burgers soak in a balsamic, tamari, Dijon and fresh herb marinade, then hit the grill until juicy and meaty. Stack them on buns with arugula, tomato and grilled red onion, and even the meat eaters at your cookout will want one.
Make the marinade: Whisk the balsamic vinegar, olive oil, mustard, tamari, coconut sugar, rosemary, thyme and garlic together in a bowl.
Wipe the mushroom caps clean with a damp cloth and remove the stems. Place the mushrooms stem side up in a shallow dish and pour the marinade evenly over them. Cover and marinate for at least 30 minutes or overnight.
Heat a grill. Remove the mushrooms from the marinade, reserving the marinade for basting. If grilling the red onion, brush the slices with a little olive oil.
Grill the mushrooms and onion for 5 to 7 minutes per side, basting occasionally with the reserved marinade. Transfer to a clean plate, keeping the mushrooms stem side down since they release liquid after grilling.
Assemble the burgers on the buns with the arugula, tomato, grilled onion and garlic aioli, if using, and serve immediately.
Notes
Make Ahead: Marinate the mushrooms overnight if you can; the longer they sit, the more flavorful they'll be.
Stovetop Method: Cook the mushrooms and onion in a grill pan or skillet over medium-high heat for about the same time, basting as you go.
Gluten-Free: Tamari keeps the marinade gluten-free, so serve on gluten-free buns. For a soy-free version, use coconut aminos instead.
Leftovers: Store cooked mushrooms in an airtight container in the fridge for up to 3 days. Reheat in a skillet or grill pan until warmed through, and toast the buns just before serving.