In a large shallow dish, add lime juice, kaffir lime leaves, garlic, and ginger. Add fish pieces and turn to coat; season with salt and freshly ground black pepper. Marinate the fish for 10 minutes.
In a large skillet over medium heat, add oil. When hot, add onion and red peppers; cook until the vegetables start to soften, about 3 minutes. Add tomatoes and hot paprika; sauté for 3 minutes. Pour in coconut milk and add chopped cilantro; stir well. Place fish pieces including the marinade including ginger, garlic and kaffir lime leaves. Gently move fish pieces around so the sauce mixture covers them. Cover and poach fish for 12 minutes but as soon as it simmers, reduce heat to low and cooking for the remaining time.
When time is up, place two pieces of fish over a bed of rice per plate; taste and adjust the sauce. Increase heat to medium-high and bring the sauce to a rapid simmer. Pour in the cornstarch mixture and stir until the sauce thickens, about 30 seconds. Spoon sauce over fish and serve immediately. Serves 4.