Combine spices, from garam masala to ground turmeric, in a bowl and mix well. Drizzle oil over the fish and cover in the spice mix, making sure you coat the fish thoroughly.
Add enough oil in a large pan to come up ¼ inch. Heat over a medium-high heat until it begins to simmer. Fry the fish, pepper, and onion, and cook, stirring occasionally, for 5-6 minutes or until the fish is cooked and the vegetables are crisp and tender. Drain the oil. Add the tikka masala and cook until heated through.
Make the raita: Combine yoghurt, cucumber, coriander, and cumin together in a bowl. Taste and add salt and pepper to taste. Place in the fridge until serving.
Serve over rice garnished with the raita on the side, crispy onions, if using, and coriander.