Combine the onion, garlic, ginger, garam masala, turmeric, coconut milk, diced tomatoes, salt and pepper in a large resealable freezer bag. Remove the excess air and seal.
Staple the two bags together above the seal.
Freeze.
Thaw.
Pour the contents of the large bag into a large skillet and cook over medium heat until the sauce begins to boil.
Continue cooking until the shrimp is pink.
Serve over rice or with Naan.