Drizzle oil on the bread slices and brown them for 3-5 minutes under a broiler till lightly browned .
In a pan or skillet, heat up the oil. Add onions, ginger and garlic to the oil all at once and saute for 2 minutes on medium heat. We dont want to brown these but the raw smell should go away.
Add the chopped tomatoes, tomato paste and green chilies next along with the spice powders & sugar. On medium heat saute for 4-5 minutes, stir a few times in between to make sure nothing is sticking to the pan. Let cook till the tomatoes cook and become jammy and you see tiny oil bubbles on sides of pan. Add coconut milk and stir for 2 minutes. Dont dry out too much.
Once the masala has cooked nicely, lower the heat and add the shrimp and salt. Gently mix so that all the shrimp is coated in masala and let cook for 3-4 minutes till the shrimp are white and cooked through. Dont rush cooking the shrimp else they will turn rubbery. You should have a saucy(not runny) masala coating the shrimp at the end of cooking.
Switch off the stove, mix in the lemon juice, chopped cilantro and bread crumbs. You can add a pinch of garam masala as well if you wish.
Spoon the shrimp on toasted bread. Add few cilantro, green chillies or scallions and serve warm.