Chayote with Onions (Chayote con Cebollas)

2 chayote squash (1 1/2 to 2 pounds total)
2 medium white onions
2 tablespoons butter or margarine
1 tablespoon vegetable oil
1/2 teaspoon dried oregano
1/4 to 1/2 teaspoon salt
Pinch of pepper

Pare and seed chayotes; cut lengthwise into 1/2-inch thick slices.

Cut onions lengthwise into halves. Cut halves lengthwise into 1/4-inch thick slices; separate into slivers.

Heat butter and oil in skillet over medium heat until foam subsides. Add onions and oregano; saut