Thaw the spinach in a sieve. Trim the leeks, split then down the centre and wash thoroughly. Cut into ½ centimetre slices. Crush, peel and chop garlic. Heat oil in a frying pan and saute the leeks and garlic for 5 minutes until soft. Remove from the heat and stir in the spinach.
Remove skin and seeds from the roasted peppers and cut flesh into 4 – 5 centimetre strips. Mix the tomatoes and tomato puree together.
Melt the butter in a saucepan. Stir in the flour and cook until frothy. Remove from the heat and stir in the milk a little at a time, stirring to remove lumps before adding more. Return to the heat and cook, stirring until the sauce boils and thickens.
Spread half the spinach mixture over the base of a xx cup deep ovenproof dish. Arrange half the red pepper over then top with lasagne, half the white sauce, half the Parmesan cheese, lasagne, tomato mixture and lasagne. Repeat the layering, finishing with the white sauce and cheese.
Bake at 200 degrees C for 30 – 40 minutes or until the lasagne is cooked.
Stand for 10 minutes before serving.