Combine lemon juice and 1 tsp curry powder in a bowl. Add fish and turn to coat.
Heat oil in a large, deep frying pan over medium heat. Cook capsicum, garlic, ginger and two-thirds of the onion, stirring, for 2 minutes or until fragrant. Add half the thyme and remaining curry powder, then cook, stirring, for 1 minute or until fragrant.
Add coconut milk and fish, then bring to a simmer. Simmer, partially covered, for 8 minutes, turning fish halfway through cooking, or until sauce has thickened and fish is cooked through.
Scatter over coriander, and remaining onion and thyme. Serve with chapattis.