Preparation
Besciamella: in a saucepan, melt butter on a medium heat. Add flour and stirring up to toast. Dilute with hot milk and broth. Keep on stirring until besciamella is thick. Add salt and pepper, season with cheese and aromatize with minced rosemary.
Anoint a baking-pan, and cover the bottom with a pasta rectangle, then spread a besciamella layer and cover with tuna chunks with lentils and thyme.
Prepare lasagna by overlapping pasta layers, besciamella, tuna chunks and thyme. Finish with a top layer of besciamella. Put lasagna in the oven at 185° and cook for 30 minutes. Complete with more tuna chunks, decorate with thyme and serve.