Heat a drizzle of oil in a large, non-stick soup pan.
Add the red onion and fry for 10 minutes until soft and starting to colour. Stir in the mango chutney and season with salt and pepper.
Fry the onions for another 3-5 minutes until dark and sticky.
Remove two thirds of the onions from the pan and set them aside.
Stir the tikka curry paste into the onions in the pan and fry for 1 minute, stirring until the onions are coated.
Add the stock to the pan with the red lentils and chopped tomatoes. Season with salt and pepper and bring to the boil.
Simmer for 15 minutes until the lentils are soft.
Blitz with a hand blender until smooth.
Whilst the soup is cooking, make the toasties.
Lay 4 slices of bread out on the side and butter them. Divide the reserved red onion between them (you can keep a little back if you want to put some on the soup). Spread the onion evenly, right up to the edges.
Top each one with grated cheddar cheese.
Butter the remaining slices of bread and place them on top of the cheese. You can then grill, airfry or pan fry the toasties until the cheese has melted and they're golden brown.
Pour the soup into bowls and top with any leftover red onion.
