Heat oil in a large pan over medium heat. Add mustard seeds and let them pop for 30 seconds.
Add onions and cook for 8-10 minutes until golden and softened.
Stir in garlic, ginger, and green chillies. Cook for 2 minutes until fragrant.
Add cardamom pods, curry powder, turmeric, cinnamon stick, and chilli powder. Cook for 1 minute, stirring constantly.
Add chopped tomatoes and cook for 5 minutes until they start to break down.
Stir in red lentils, stock, and pineapple chunks. Bring to a boil.
Reduce heat to low, cover partially, and simmer for 20-25 minutes until lentils are soft and breaking down.
Add prawns and lemon juice. Cook for 3-4 minutes until prawns are pink and cooked through.
Season with salt to taste and remove cinnamon stick.
Serve hot garnished with fresh coriander and red chilli flakes alongside basmati rice or naan bread.