In a medium bowl, combine garam masala, ginger, garlic, scallion whites, 1/2 teaspoon salt and 1/4 teaspoon pepper. Add shrimp and toss well. Heat a large skillet over high heat, then add the oil. Add the tomatoes and 1/4 teaspoon salt. Cook until the whole tomatoes burst and the halved ones start to wrinkle, about 2 minutes.