Melt butter and add sugar; stir and cook over low heat for a few minutes. Add about a tablespoon of water, then pineapple with juice. Dissolve cornstarch in a little water and add to the mixture. Stir in lemon juice. Cut toast into small cubes and add. Place in greased 13x9-inch baking dish and bake at 350 degrees for 30-45 minutes.