Preheat oven to 375 degrees F. Grease a 9×13 inch casserole dish.
Heat 2 tablespoons of butter in a large skillet over medium heat. Stir the 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.
Melt remaining 2 tablespoons butter in clean skillet; stir in panko until all crumbs are coated; reserve.
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.
Mix all of the ingredients together in a large bowl and pour into greased casserole dish. Top with the buttered panko crumbs, the reserved Gruyere, and Parmesan cheese.
Bake in the preheated oven until hot throughout and lightly browned, about 30 minutes. Let sit 10 minutes before serving.
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