"My Indian mother-in-law's recipe. It is my favorite way to eat shrimp now. The spice measurements are approximate because of course she doesn't measure, so adjust to your liking. Indians will also make this dish with potatoes (which need to be boiled ahead) or cauliflower instead of green pepper. It would work with just about any veggie you like with shrimp."
Mix paprika, cumin, turmeric, and salt in a small bowl. Spread 3/4 the spice mixture over the shrimp in a bowl; set aside for about 10 minutes.
Crush onion slightly with a mortar and pestle. A few hard jabs is good; onion should be moist but not watery.
Heat oil in a frying pan over high heat; add mustard seeds and cook until crackling subsides, 1 to 2 minutes. Reduce heat to medium-high. Cook and stir onion, garlic paste, and ginger paste into the mustard seeds until onion is browned, 10 to 15 minutes.
Stir water into remaining spice mixture; pour into pan. Cook and stir tomatoes in onion mixture until softened, about 5 minutes. Add green peppers and cook for 1 to 2 minutes.
Reduce heat to medium; add shrimp and cook until no longer pink, 3 to 5 minutes. Increase heat to medium-high and stir until all liquid evaporates. Remove from heat. Sprinkle with black pepper and lemon juice.