Wash the shrimp thoroughy, then 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 2-3 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. When it is limp add the chopped tomatoes, about 3-4 tbsp. water, and season with salt and pepper to taste. Allow to cook over moderate heat, stirring occasionally. Correct seasoning and add ground cumin and cilantro. Cook 3 minutes, covered. If sauce seems too dry, add a few more tbsp. water, or even seafood broth if you've got it. Add the shrimp, stir well, and cover, but remove from heat. Let stand 5 minutes before serving.