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.
Add shrimp mixture and sauté for another 2 to 4 minutes, turning the shrimp, until they are pink all over.
Gently stir in butter and cook for another minute, until the shrimp are cooked through. Remove from heat and add a squeeze of fresh lime juice. Taste and add more salt and spices, if needed. Garnish with mint and scallion greens and serve immediately.