Whisk together the garlic, balsamic vinegar, and olive oil in a small bowl. Place the mushrooms and thyme in a large Ziploc bag and pour the vinaigrette over the mushrooms. Let sit at room temperature for an hour. (You can let it marinate for 30 minutes if you’re in a hurry.)
Meanwhile, roast the peppers: if you have gas burners, place each pepper on a separate burner and turn the flame to high. Let cook for 8 to 9 minutes, turning about every 2 minutes with tongs, until completely charred on the outside. Turn off heat, place the peppers in a large heatproof bowl and cover with plastic wrap to steam for 15 to 20 minutes.
When the peppers are cool enough to handle, remove the burnt skins. Cut the peppers in half and remove the seeds. (If you need to, you can hold the peppers under running water to help remove the skin and seeds, but it will take away some of the flavor!) Cut the peppers into thin strips.
Heat a grill pan over high heat and brush with 1 tablespoon of oil. Remove the mushrooms from the bag and season on both sides with salt and pepper; discard the thyme sprigs pour the marinade into a large bowl. Grill for 3 to 4 minutes per side, or until softened. Let cool slightly, then cut into strips. Place the roasted peppers and the grilled mushrooms in the bowl with the reserved marinade and let sit for 10 minutes.
In a large bowl, combine the mixed greens, roasted peppers, onions, mushrooms, and remaining marinade and toss to combine. Season with salt and pepper.
Serve the salads on plates, topped with goat cheese and pine nuts (if desired).
*You can try many different cheeses in this salad: though I love goat cheese, it’s also delicious with blue cheese or shaved Parmesan or Manchego.
NOTE: If you want to make this into a heartier main course salad, it’s delicious with some grilled chicken or steak. Just make some extra Balsamic marinade to marinate the meats as well (in a separate bag!).