Ingredients
2 tablespoons unsalted butter
1 white onion, thinly sliced (2 cups)
3/4 teaspoon dried thyme
Kosher salt and freshly ground pepper
2 tablespoons Dijon mustard
1/2 cup beer, such as lager; or dry white wine, such as Sauvignon Blanc
6 ounces sharp cheddar or Gruyère, or a combination, grated (2 cups)
4 ciabatta hoagie or grinder rolls, split
1 1/4 pounds thinly sliced roast beef
1 cup giardiniera, or a combination of giardiniera and dill or sour pickles, roughly chopped
Preparation
In a medium skillet, melt butter over medium-high heat until foamy. Add onion and thyme, season with salt and pepper, and cook, stirring, until onion is translucent, 5 to 6 minutes. Stir in mustard, then beer. Bring to a boil and cook until liquid has mostly evaporated, about 30 seconds. Divide onion mixture into 4 piles; top each evenly with cheese. Reduce heat to low, cover, and cook until cheese has melted, 1 to 2 minutes.
Meanwhile, scoop out some of top of each roll to make room for filling. Using a spatula, transfer cheese-onion piles to bottom halves of rolls. Top with roast beef and giardiniera. Sandwich with scooped-out roll tops and serve immediately.
