Cut the onion into slices and peppers into strips. Put salt, garlic, ginger and chili powder in a small bowl. Put the lemon juice and stir to make a paste. Place frozen shrimp in a bowl with cold water and let them thaw. Heat the olive oil in a medium heat pan, add the onion and fry until golden brown. Reduce the heat, add the spice paste to the onions and fry for 3 minutes while stirring. Add peppers and tomatoes with juice and cook 5-7 minutes, stirring occasionally. Place frozen shrimp in the pan and simmer the mixture for about 3-4 minutes, stirring occasionally. Garnish with coriander and serve hot. Serve garnished with boiled rice.