For the mushrooms:
10 ounces brown or cremini mushrooms, tough stems removed, coarsely chopped
2 tablespoons butter, plus more for buttering the biscuits
1 medium shallot, minced
1 teaspoon chopped fresh thyme leaves
1/2 teaspoon fine salt
1/4 teaspoon freshly ground black pepper
1/2 cup sour cream

For the sandwiches:
4 biscuits, split in half
1/4 cup shredded Gruyère cheese
1 cup arugula