Carina. What you have is yummy. If it were me, I’d chunk them up and saute them in some olive oil or butter along with the minced garlic, throw in some chopped tomatoes, and basil and/or spinach if you have any. Dish it out and sprinkle feta over the top with a light drizzle of the balsamic.
Or mix up the feta and garlic with some bread crumbs, turn the shrooms bottom up and mound some of the stuffing on them, cover with more bread crumbs and drizzle with oil. Broil them until the stuffing is crusty, then drizzle the balsamic over them.
Turbo’s idea sounds perfect. If you chop them up instead of broiling, serve it over rice, with wild rice too if you have it.
Too late for a tonight thang, but whole portobello caps are great marinaded in olive oil, balsamic vinegar, tamari, honey, sherry, pepper, ginger and whatever other herbs at hand. Marinate overnight, and grill like a burger, basting with the marinade. Yee-ha!