Prepare the Bechamel Sauce by melting the butter on the heat and gradually adding 4 tbsp flour, while stirring nonstop. Pour in the milk in a thin trickle, stir until thickened. Season with salt and black pepper.
Heat the oil and saute the mince. Add the tomato concentrate + 1 cup water, leave it to simmer. To thicken it, dissolve 1 tbsp flour with a little water and pour this in.
Pour part of the Bechamel sauce at the bottom of a tray with a nonstick coating, arrange a layer of lasagna sheets, layer of tomato sauce with mince. Sprinkle grated gouda, pour cream and keep alternating until you've arranged all the sheets.
Pour on the remaining Bechamel sauce on top and bake until golden. Before taking the tray out, arrange slices of tomato and grated gouda. Bake until the latter melts. Cut into slices and serve warm.