Saute onions until soft.
Add pepper. Fry 2-3 mins. (can omit pepper if you wish)
Add courgettes, mushrooms, tinned tomatoes with juice, herbs, garlic & seasonings.
Stir well & simmer uncovered 15 mins.
Stir in tomato puree & cook a further 15 mins until mixture is thick. Taste & adjust seasoning.
Melt butter, add flour, stir over low heat 1-2 minutes. Remove from heat & gradually add milk.
Return to heat & simmer until thick.
Add half of the cheese, salt & pepper.
Dip sheets of lasagne into hot water before you use them. Do this one by one or they get sticky.
Arrange vegetable mixture, lasagne & cheese sauce in that order in large oblong dish, ending with the sauce (I always make two layers).
Sprinkle remaining cheese on top and put tomato slices on top of cheese.
Bake at 200c degrees for 45 mins until golden and bubbling.
Serve with salad and crusty bread.