1 tablespoon extra-virgin olive oil
1 small onion, halved and sliced
1 tablespoon finely chopped fresh oregano
1/2 cup white wine or reduced-sodium chicken broth
1 pound green beans, trimmed
1 medium summer squash or zucchini, halved and cut into 1-inch pieces
1 cup halved cherry tomatoes or grape tomatoes
1/4 teaspoon salt
1/4 teaspoon freshly ground pepper
1/4 cup finely shredded Parmesan cheese