Green Bean Casserole - Alison Roman

Ingredients

• 2 tablespoons olive oil
• 1 medium onion , thinly sliced
• Kosher salt , freshly ground black pepper
• 1 pound mushrooms , such as cremini, button, oyster or maitake, quartered
• 1 pound green beans , stems trimmed, cut in half or coarsely chopped
• 2 tablespoons unsalted butter
• 2 tablespoons flour
• 1 ¾ cups whole milk
• 2 tablespoons soy sauce or tamari
• 1 clove garlic , finely grated or chopped
• An undetermined amount of French’s Onions , from the container