1 15-ounce can kidney beans, drained and rinsed
1 small red onion, shredded and squeezed dry (about 1/2 cup)
1/3 cup chopped walnuts
1 small carrot, finely diced
1 cup plain or whole-wheat breadcrumbs
2 scallions, white and green parts, finely chopped
2 tablespoons chopped fresh parsley
2 teaspoons Worcestershire or soy sauce
Kosher salt and freshly ground pepper
3 tablespoons extra-virgin olive oil
3 cups assorted mushrooms, sliced
4 English muffins, split and toasted
Mayonnaise and/or mustard, for garnish
Baby spinach or arugula, for garnish