Preheat oven to 400 degrees. Sauté onion and green pepper in oil in a large skillet over medium heat until onion is golden. Add beef and cook until browned. Drain off fat. Dissolve bouillon cube in boiling water. Add to cornstarch mixture, then stir into skillet along with salt, pepper and steak sauce. Heat and stir an additional 2 minutes. Spoon into ungreased 1 1/2 - quart casserole. Spread potatoes over the surface and roughen with a fork. Bake, uncovered, for 25 to 30 minutes, then broil 4 inches from heat for 2 to 3 minutes to brown top. Casserole may be prepared ahead of time, then covered and refrigerated until ready to use. Just bake it for a few more minutes.