Boil some water in the kettle and keep aside.
Heat the oil in the frying pan and fry the onions and garlic on medium heat until they become soft. Add the black pepper.
Add the tomato puree and mix through then add the tinned lentils, tomatoes and celery and mix together. Add some water from the kettle to loosen the sauce but not too runny.
Add the powdered stock, paprika, ginger, cayenne pepper, turmeric and tomato ketchup. Bring to the boil, then simmer for 10 minutes.
Preheat oven to 180℃ Fan/200℃/400℉
Meanwhile, make the cheese sauce. Heat the oil and butter in a saucepan until melted, and add the corn flour. Using a whisk, mix quickly and slowly add some warmed milk. Keep mixing until thickened like sauce and take off the heat. Add the mustard, salt and pepper and grated cheese. Mix through. If it feels a bit thick add some more milk.
Place half of the lentil mixture on the bottom of the oven-proof dish and spread it out. Put the lasagne sheet on top but not overlapping. You may need to break some, so they fit.
Next, add a layer of cheese sauce and spread it out, covering the sheets. Add another layer of lasagne sheets.
Add the remainder of the lentil mix and spread out again and add the last layer of lasagne sheets. Top with the final layer of cheese sauce and sprinkle the top with grated cheese.
Bake in the oven for 40 minutes.