Legendary Green Bean Casserole

Name: Legendary Green Bean Casserole
Ingredients: 2 large white onions, sliced thin; 1/2 cup whole milk; Vegetable, oil for frying; 1 cup all-purpose flour; 2 teaspoons seasoning salt; 1 1/2 pounds trimmed fresh green beans; Kosher salt and freshly ground black pepper; 2 tablespoons olive oil; 4 tablespoons salted butter; 8 ounces cremini mushrooms, sliced; 1 medium white onion, diced; 4 cloves garlic, sliced; 1 tablespoon chopped fresh rosemary; 1/4 cup all-purpose flour; 2 1/2 cups chicken stock; 1/2 cup heavy cream; 1/3 cup freshly grated Parmesan; Fresh parsley leaves, for garnish