Wash the shrimp thoroughly, then shell and devein. Wash again and dry in peper towels. Stir-fry in the butter in a medium skillet, with the mashed garlid added, for 2 or 3 minutes. Add a little salt and pepper but be careful if you're using salted butter. Set aside.

In a large skillet heat your oil and saute the sliced onion, stirring constantly. When onion is limp, add the chopped tomatoes with about 3 or 4 tbsp. water and season with salt and pepper to taste. Allow to cook over moderate heat, stirring occasionally, and correct seasoning before adding the crushed oregano. Cook 3 more minutes, covered, and add the shrimp and the chile slices.