Portabella Mushroom and Vidalia Onion Gratin

Name: Portabella Mushroom and Vidalia Onion Gratin
Prep time: 20 min
Cook time: 30 min
Total time: 50 min
Cuisine: American, Southern
Ingredients: 1 pound baby portabella mushrooms (sliced); 1 extra large vidalia onion or 2 medium (sliced into small wedges); olive oil; 1 tsp salt; 1/2 tsp freshly ground black pepper; 4 Tbsp butter (divided); 3 cloves garlic (minced); 2 tsp fresh thyme; 1 Tbsp red wine vinegar; 3 finely chopped green onions; 2 1/2 Tbsp all purpose flour; 1 1/2 cups whole milk; 2 cups shredded Muenster cheese (divided); 2 Tbsp chopped Italian parsley (divided); 1 1/2 cups fresh bread crumbs; 2 Tbsp grated Parmesan cheese