INGREDIENTS
3 tablespoons Dijon mustard
2 tablespoons mayonnaise
4 large portobello mushrooms, stemmed
2 tablespoons olive oil
Kosher salt
Freshly ground black pepper
4 ounces smoked Gouda cheese, grated (about 1 cup)
4 brioche hamburger buns, split
4 ounces peppadew peppers, thinly sliced (about 1/2 cup)
1 cup loosely packed arugula

INSTRUCTIONS
Heat an outdoor grill to medium-high heat (375°F to 425°F). Whisk together the Dijon and mayonnaise in a small bowl and set aside.
Brush mushrooms on both sides with the oil, then season with salt and pepper. Place the mushrooms on the grill stem-side down, cover, and grill for 4 minutes. Flip the mushrooms, cover, and grill until mushrooms are tender, about 4 minutes more.
Divided the Gouda among the mushroom caps. Arrange the buns cut-side down on the grill. Cover and cook until the cheese is melted and the cut sides of the rolls are golden-brown, about 1 minute more.
Place the bottom halves of the buns cut-side up on plates and spread with half of the Dijonaise. Top each with a mushroom. Top with the peppadews and arugula. Spread the cut side of the bun tops with the remaining Dijonnaise, place on the arugula to close the burgers, and serve.

RECIPE NOTES
No-grill option: You can also bake the mushrooms instead of grilling them: Heat the oven to 425°F. Place the oiled and seasoned mushrooms on a rimmed baking sheet stem-side down and roast until just tender, about 15 minutes. Flip them, fill the caps with the Gouda, and continue to bake until the cheese is melted, 2 to 3 minutes. Toast the buns in the oven.