Slow-Cooker Green Bean Casserole with Crispy Onions

To prepare green beans & mushrooms: Combine green beans, mushrooms, chopped onion, garlic, oil, 1 1/4 teaspoons salt and 1/2 teaspoon pepper in a 5 1/2- to 6-quart slow cooker. Cook on High until tender, about 2 1/2 hours.

Meanwhile, preheat oven to 400 degrees F.

To prepare onion topping: Combine sliced onion and buttermilk in a medium bowl. Let stand for 15 minutes. Drain (discard buttermilk) and return the onion to the bowl. Sprinkle with flour, pepper and salt and toss to coat well. Transfer to a large rimmed baking sheet. Generously coat with cooking spray. Bake, stirring once or twice, until tender and crisp, 25 to 28 minutes. Let stand at room temperature for up to 2 hours.

To prepare sauce: When the vegetables are done, combine cornstarch and sherry in medium bowl; stir until smooth. Add sour cream and stir to combine well. Add the sour cream mixture and thyme to the vegetables; stir until well coated with the mixture. Cover and continue cooking for 20 more minutes. Serve topped with the crispy onions.