1 lb. mixed wild, shiitake, and/or crimini mushrooms, broken into pieces or sliced 1/4" thick (about 4 cups)
3 garlic cloves, peeled, thinly sliced
6 thyme sprigs
1/4 tsp. crushed red pepper flakes
1/4 cup plus 2 Tbsp. extra-virgin olive oil, plus more for drizzling
1 3/4 tsp. (or more) kosher salt, divided
1 medium onion, finely chopp...