Peel and chop onions and potatoes into chunky bite-sized pieces, Saute onions and potatoes in butter until onions are tender. Drain the juice from the 5 cans of clams into the potatoes and onions, reserving the clams to add later. Add water, just to cover potatoes and onions. Cover and cook on low heat until potatoes are tender. blend in the tomatoe soup, milk,and clams. Add salt and pepper to taste. Heat until just boiling.