- 4 Tbsp. (1/2 stick) unsalted butter, divided
- 1 sweet yellow onion, sliced into 1/4-inch-thick rounds
- 1/2 tsp. kosher salt
- 2 Tbsp. balsamic vinegar
- 1/2 tsp. sugar
- 4 slices whole grain sandwich bread
- 1 Tbsp. mayonnaise, at room temperature
- 1 cup grated Gouda (cheddar, Havarti, Gruyère and taleggio all work, too)
- 1 semiripe pear, peeled, halved, cored and thinly sliced
- Ground black pepper
Total time: 20 minutes
In a medium nonstick or stainless skillet, melt 2 Tbsp. butter over medium heat. Add onion and salt and cook, stirring often, until onion begins to soften and caramelize but retains some texture, about 6 minutes. Add vinegar and sugar, and let liquid reduce and coat onion, about 2 minutes.
Spread one side of each bread slice thinly with mayonnaise. Turn 2 slices of bread mayo side down and sprinkle with half of cheese. Add a few pear slices and cracks of pepper, top each with a quarter of onion and sprinkle with remaining cheese. Set 2 other bread slices on top of cheese, mayo side up. Remaining onion can be refrigerated in a sealed container up to 3 days or frozen up to 2 months.
Wipe out skillet and melt remaining 2 Tbsp. butter over medium heat. Place sandwiches in skillet and reduce heat to medium-low. Set a large, heavy plate or pan on top of sandwiches. Cook until one side is golden brown, 2 to 3 minutes.
Carefully remove plate (it might be hot) and use a spatula to flip sandwiches. Replace plate and toast second side until golden brown, another 2 to 3 minutes. Transfer sandwiches to a plate, cut in half and serve.