Heat some oil in a large pan and fry half of the sliced onions for about 5 minutes until starting to soften. Add the curry powder and crumble in a stock cube. Stir for a minute until fragrant.
Tip in your red lentils. Then, add 200ml of creamy coconut milk and 160ml of water. Stir until combined.
Cover with a lid and cook for 15-20 minutes on low heat, stirring occasionally.
In the meantime, prepare your mushrooms for grilling. Thickly sliced mushrooms are added onto a baking tray in a single layer. Scatter with the rest of the sliced onions. Drizzle some olive oil and sprinkle with salt. Cook under the grill for 7-10 minutes. Keep your eye on them though and don't let the mushrooms burn. Your onions will turn crispy and may char a bit and that's ok.
In the meantime, prepare your coriander relish. Add finely chopped garlic, ginger, coriander, chilli flakes, caster sugar and a tbsp of rice vinegar into a pestle and mortar and crush until you end up with a rough paste.
Transfer the mixture to a small bowl, add a pinch of salt, 3 tbsp of olive oil and mix.
When your dahl is cooked, divide it between two bowl. Top with your grilled mushrooms and onions. Serve with dollops of coriander relish.