Brown mince in a pan till edges start to crisp. Add onion, carrot, red pepper and garlic. Stir occasionally till softened.
Add the lentils and stir till coated, then add the courgette, tomatoes, water, herbs, chilli, half the salt and pepper. Simmer for 10 to 15 minutes uncovered, allowing to reduce slightly.
Meanwhile, make the bechamel (white sauce): In a pan, combine the butter and flour. Stir over a medium heat till it looks a bit frothy, 1 to 2 minutes.
Add milk slowly using a whisk to prevent lumps. Add the mustard powder. The sauce will be runny. Remember the lasagne sheets are starchy and will help the whole thing set up. Set aside 1/3 of the white sauce for the final layer.
By this point the lentils in the meat sauce should be soft and broken down. That is the perfect indication that it's ready.
Layer the white sauce first, just enough to coat the baking dish. Add the meat sauce straight on top, then lasagne sheets. Repeat the layering 3 to 4 times. When the last lot of lasagne is down place over the reserved 1/3 of the white sauce.
Cover with tin foil and set aside for 2 hours, or overnight in the fridge. This will allow the pasta sheets to soften and the whole thing to set up before cooking. this will mean a firmer texture, and no crunchy lasagne sheets.
Bake for 30 minutes at 180 C / Gas 4. Remove foil and place on mozzarella, cook for a further 15 minutes. Remove from the oven and sprinkle on the Parmesan cheese.
Use 1 tablespoon of fresh chopped herbs instead of dried, if desired.
Before serving allow to rest uncovered for 15 to 30 minutes, this will allow it to cool to a non mouth-burning temperature, and also firm up the texture. This is a great time to bake frozen garlic bread or make a salad as an accompaniment.