French Onion Green Bean Casserole

Recipe View 5 mins Preheat the oven to 375 degrees F (190 degrees C). Grease a 9x13-inch casserole dish.

Recipe View 15 mins Heat 2 tablespoons of butter in a large skillet over medium heat. Stir onions into the pan; cook and stir until very soft and golden brown, about 15 minutes. Stir in sherry vinegar. Transfer to a small bowl, and reserve.

Recipe View 6 mins In the same skillet, melt 2 more tablespoons butter. Whisk in 2 tablespoons flour. Stirring constantly, cook over medium heat until flour smells like baked pie crust, about 3 minutes. Slowly whisk in milk, about 1/2 cup at a time, then whisk in nutmeg, cayenne pepper, and thyme. Bring to a simmer, and cook until thickened, whisking constantly, 3 more minutes. Transfer to a bowl and reserve.

Recipe View 2 mins Melt remaining 2 tablespoons butter in clean skillet; stir in panko until all crumbs are coated; reserve.

Recipe View 4 mins Bring a large saucepan of salted water to a boil over medium heat, and boil beans until bright green and crisp but not raw tasting, 3 to 4 minutes. Drain very well.

Recipe View 8 mins Spoon half of the caramelized onions into the prepared casserole dish; top with green beans. Season with salt and black pepper. Spread reserved white sauce over the green beans; sprinkle with half of the Gruyere cheese. Top with the rest of the onions, and use the back of a spoon to push onions down into sauce and cheese. Top with the buttered panko crumbs, the remaining Gruyere, and Parmesan cheese.

Recipe View 40 mins Bake in the preheated oven until hot throughout and lightly browned, about 30 minutes. Let sit 10 minutes before serving.