Preheat oven to 375°F. Grease an 8x8-inch baking dish with non-stick cooking spray.

Set a large skillet over medium heat and add butter. When melted, add onions, salt, and pepper. Cook for 10 minutes, stirring often, until soft and lightly caramelized. Stir in the sherry and cook 1 minute more. Transfer onions to a bowl and set aside.

Bring a large pot of salted water to a boil. Add green beans and cook for 3 minutes, until bright green and just crisp-tender. Drain well and set aside.

Return the skillet to medium heat and melt butter. Whisk in the flour and cook 1 minute. Slowly whisk in half-and-half. Add nutmeg, cayenne, and thyme. Lower heat and simmer, whisking frequently, until slightly thickened. Remove from heat and stir in half the Gruyere until melted.

Add the cooked green beans to the skillet and stir gently to coat.

In a medium bowl, combine breadcrumbs, Parmesan, and melted butter.

Transfer the green bean mixture to the prepared baking dish. Sprinkle with the remaining Gruyère. Top with caramelized onions and the breadcrumb mixture.

Bake for 20 minutes. Loosely cover with foil and continue baking 10 minutes, until heated through.