How to make Easy Green Bean Casserole

Ingredients • 3 tbsp butter, divided • 1/2 cup yellow onion, chopped • 8 oz fresh Bella mushrooms, sliced • 1 tbsp all-purpose flour • 2 tbsp minced garlic • 1/4 cup chicken stock • 1 cup whipping cream • 10 oz cream of mushroom soup • 1/2 cup grated Parmesan cheese • 1 cup shredded Monterey jack cheese • 1 tsp sea salt, divided • 1 tsp black pepper • 1/4 tsp ground allspice • 1/2 tsp ground thyme • 3 cans french-cut style green beans, drained • 1 1/2 cups crispy fried onions, divided