Tomato Frittata

2 tablespoons olive oil
2 medium onions, thinly sliced
1 (15 ounce) can of Italian tomatoes,
    drained and coarsely chopped
6 eggs, beaten
Salt and freshly-ground pepper, to taste
3 tablespoons freshly-grated Parmesan cheese
1/2 cup roughly chopped fresh basil

Heat the olive oil in a large, heavy skillet over moderate heat and saut