Boil peeled potatoes until not quite done. Cool and then cut into cubes. Melt butter in large skillet and saute onion, then potatoes. Remove onion and potatoes and place in bowl for the time being. Heat oil and toss in peppers. Saute briefly and add broccoli. Saute and then add mushrooms. Saute tomatoes briefly. Return the onion mixture to skillet and season with soy sauce, salt and pepper. Turn off and top with cheese if desired. Note: vegetable broth may be substituted for some of the butter and oil if