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

In a large skillet heat the oil and saute sliced onion, stirring constantly. When onion is limp, add the chopped tomatoes with 3 or 4 tablespoons of 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.

Remove from heat and serve in the skillet or a heated serving dish. Remove the chile slices and save for the chile eaters.