Grilled Ham & Gouda Sandwiches With Frisee

1 tablespoon extra-virgin olive oil
1 large onion, thinly sliced
Unsalted butter, room temperature
4 1/3-inch-thick slices country white bread or sourdough bread
4 ounces thinly sliced smoked ham, divided
3 ounces Gouda cheese, thinly sliced, divided
2 large handfuls Red Dog Farm frisee, torn into bite-size pieces (about 1 cup), divided

Heat oil in large nonstick skillet over medium heat. Add onion; sauté 5 minutes. Reduce heat to medium-low; cover and cook until onion is very tender and golden, stirring frequently, about 25 minutes longer. Season with salt and pepper. Cool slightly.

Butter 2 bread slices. Place bread, buttered side down, on platter. Divide onion, ham, cheese, and frisee between bread slices. Top each with another bread slice; butter top slices.

Heat another large nonstick skillet over medium heat. Add sandwiches and cook until bread is golden brown and cheese melts, pressing occasionally with spatula, about 4 minutes per side. Cut sandwiches in half diagonally and serve immediately.