2 teaspoons extra-virgin olive oil
1 medium onion, sliced
4 large portobello mushrooms, stems and gills removed (see Tip), sliced
1 large red bell pepper, thinly sliced
2 tablespoons minced fresh oregano, or 2 teaspoons dried
1/2 teaspoon freshly ground pepper
1 tablespoon all-purpose flour
1/4 cup vegetable broth, or reduced-sodium chicken broth
1 tablespoon reduced-sodium soy sauce
3 ounces thinly sliced reduced-fat provolone cheese
4 whole-wheat buns, split and toasted