Ingredients

• Ingredients
  1. 6 large Portobello mushrooms (6 ounces each), stemmed and stems reserved
  2. 2 tablespoons extra-virgin olive oil, plus more for brushing
  3. 1 yellow bell pepper, finely chopped
  4. 2 medium shallots, minced
  5. 2 garlic cloves, minced
  6. 1/3 cup coarsely chopped roasted salted cashews
  7. 1/4 cup finely chopped dill or basil
  8. 4 ounces aged Gouda or Cheddar cheese, shredded (1 cup), plus 4 thin slices aged Gouda or Cheddar
  9. Kosher salt
  10. Cayenne pepper
  11. 1/2 cup mayonnaise
  12. Freshly ground black pepper
  13. 4 kaiser rolls, split
  14. 1 small bunch of arugula
  15. 4 thin tomato slices

Directions

1. Brush the underside of 4 of the Portobello mushrooms with olive oil. Cut the stems and the remaining 2 mushroom caps into 1/2-inch dice. Transfer the diced mushrooms to a food processor and pulse just until finely chopped.
2. In a large skillet, heat the 2 tablespoons of olive oil until shimmering. Add the bell pepper, shallots and half of the garlic and cook over moderate heat, stirring, until softened and beginning to brown, about 5 minutes. Add the chopped mushrooms and cook over high heat, stirring, until softened and all of the liquid has evaporated, about 5 minutes longer. Add the cashews and half of the dill and transfer to a bowl to cool. Stir in the shredded cheese and season with salt and cayenne.
3. In a small bowl, combine the mayonnaise with the remaining minced garlic and dill and season with black pepper.
4. Light a grill. When the fire is medium hot, brush the grate with oil. Grill the mushrooms, stemmed side down, for 7 to 8 minutes, or until softened and browned. Brush the tops with olive oil and transfer them to a platter, stemmed side up. Pack the filling into the mushroom caps in a slightly flattened mound. Top with the cheese slices.
5. Return the mushrooms to the grill, filling side up. Cover and grill over a medium-hot fire until the cheese is melted and the Portobello bottoms are browned and cooked through, 5 minutes. Grill the cut sides of the rolls until toasted. Spread 2 tablespoons of the mayonnaise on the rolls' cut sides. Set the mushrooms on the bottom halves, top with the arugula and tomatoes and serve, with the roll tops alongside.