Just like the standard American green bean casserole, this one smothers green beans in creamed mushrooms, then tops them with crunchy fried onions. To re-create the classic creamy soup that coats green beans, I simmer fresh mushrooms in a sauce made with oat milk. The natural sweetness of the oat milk mimics the sugary quality of canned soup. To save time and oven space, this dish doesn’t even need to be baked.
Bring a large pot of water to a boil. Add the green beans and 1 tablespoon salt. Boil, stirring occasionally, until brighter green and tender with a little bite left, about 5 minutes. Drain, rinse under cold water until room temperature, and drain again.
Meanwhile, heat 1/4 cup oil in a large Dutch oven over medium-high heat. Add the onion and a big pinch of salt and cook, stirring often, until almost translucent, 5 to 10 minutes.
Add the mushrooms, the remaining 2 tablespoons oil and a pinch each of salt and pepper. Cook, stirring often, until the mushrooms soften, about 5 minutes. Add the flour, reduce the heat to medium, and cook, stirring, until the raw flour smell dissipates, 2 to 3 minutes. Add the oat milk, 1/4 cup at a time, stirring well to incorporate each addition before adding the next. You want the sauce to stay smooth and not get lumpy. Bring the sauce to a gentle simmer and cook until creamy, about 3 minutes. Season to taste with salt and pepper.
Return the green beans to the pot in which they cooked and add the mushroom sauce. Fold until evenly coated, then transfer to a serving platter. Sprinkle the fried onions all over the top right before serving.
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