A classic American comfort dish featuring tender green beans in a creamy mushroom sauce, topped with crispy fried onions. This homemade version uses fresh ingredients for a richer flavor.
Preheat oven to 350°F (175°C). Blanch green beans in boiling salted water for 3 minutes, then transfer to ice water. Drain well.
In a large skillet, melt butter over medium heat. Add mushrooms and onion; cook until mushrooms release their liquid and start to brown, about 6-8 minutes. Add garlic and cook 1 minute.
Sprinkle flour over the mushroom mixture and stir constantly for 1 minute. Gradually whisk in milk and cream. Cook, stirring, until sauce thickens, about 3-4 minutes. Season with salt, pepper, and nutmeg.
Remove from heat. Stir in green beans and 1 cup of crispy fried onions. Transfer to a 2-quart baking dish.
Top with remaining 1/2 cup crispy fried onions. Bake for 20-25 minutes until bubbly and golden. Let stand 5 minutes before serving.
Tips
For a make-ahead option, assemble the casserole without the top onions, refrigerate, and add onions just before baking.
Substitute frozen green beans (thawed and drained) if fresh are unavailable.
Leftovers can be stored in an airtight container in the refrigerator for up to 3 days.