Preheat the oven to 180°C.
Cut the tomatoes in half and put them in a baking tray, cut-side facing upwards.
Peel then cut the onion into wedges and tuck these between the tomatoes. Add the garlic and chilli (unpeeled). Mix the spices and scatter over the vegetables. Season with salt and pepper. Drizzle with 2 Tbsp vegetable oil. Bake in the oven for 25 minutes, then let it cool. (You can refrigerate it until need it).
Rinse and drain the red lentils and put them in a large saucepan. Add the water, the turmeric root (or 1 tsp turmeric powder), ginger, garlic clove, bay leaf, cinnamon bark and salt. Bring to the boil, then simmer until the lentils are cooked and become mushy. The soup should be quite thick. Remove the cinnamon and bay leaf.
Squeeze the garlic cloves out of their skin. Remove the chilli stalk. Add the garlic & chilli flesh to the dal, together with the the roasted tomatoes, including the cooking jus. Add the spinach if used, bring to a simmer, then put the lid on and rest for 5 minutes.
Add the fresh coriander leaves. Using a hand-held blender, whiz for a few second to thicken the curry - but don’t blend it into a smooth soup.
Warm it up if needed and serve with lime wedges.