Preheat skillet(non-stick) and saute onion in oil. Add mushrooms and saute about 2 minutes, stirring constantly, until mushrooms wilt. Spread half of fillet slices on mushrooms; sprinkle with salt and pepper; and remaining fillet; sprinkle these with salt and pepper. Add celery, soy sauce, and sherry; cook gently, covered, for ten minutes. Food Exchange per serving: 1 VEGETABLE EXCAHNGE + 3 LEAN MEAT EXCHANGES + 1 FAT EXCHANGE CAL: 200