Clean mushrooms and remove the stem. Pat dry and set aside.
Whisk together olive oil, balsamic vinegar, salt, pepper, thyme and garlic together in a small bowl. Using a pastry brush, fully coat the mushrooms until all of the marinade is used up. Set aside.
Remove several roasted red onions from the jar and set on a paper towel to drain. Set aside.
Preheat your grill to medium high heat, allowing your mushrooms to continue to marinate while the grill gets hot.
Once the grill has been preheating for around 5-10 minutes, add mushrooms top side down and grill with the lid closed over medium high for approximately 3-4 minutes.
Flip your mushrooms to the other side and grill for an additional 2-3 minutes until tender. Immediately remove from heat.
(Optional) While mushrooms are grilling melt some butter in a nonstick skillet over medium/medium high heat. When the pan is hot place your buns cut side down in the skillet and toast for a minute or two until crisp. Remove from the heat and set aside.
Assemble your burgers by spreading approximately 1 tsbp of goats cheese one each bun, top with baby spinach, a few slices of red onion, a few slices of roasted red pepper, grilled portobello mushroom, more baby spinach and top bun. Enjoy!