Heat enough vegetable oil to deep-fry in a large saucepan to 180˚C. Once hot, deep-fry the brussels sprouts and red onion segments for 3 minutes, or until well-browned, taking caution as the sprouts will spit heavily for the first 30 seconds – keep a pot lid handy! Use a slotted spoon to remove the vegetables from the hot oil to a wire rack. Deep-fry some curry leaves if you like, to garnish the curry later on.
Heat 2 tbsp vegetable oil in a second large saucepan over medium heat. Once hot, add the mustard seeds, curry leaves and chilli and cook, stirring for 1 minute.
Add the onion, ginger and garlic to the pan, then cook, stirring, for 2 minutes further.
Add the ground spices and tomato to the pan and bring to a simmer. Cook for 5 minutes, or until the tomato has broken down and thickened.
Stir the fried vegetables through the curry and season to taste with salt.
Spoon the brussels sprouts curry onto a large serving plate and sprinkle with deep-fried curry leaves (if using). Serve with lemon wedges on the side.
