For the ragù:
Finely chop 1 small onion, 1 garlic clove, 1 carrot and ⅓ stalk of celery and sautè in a pan with 2 tablespoons of olive oil.
Add the ground beef and cook for a few minutes until brown.
Add the passata, season with salt and simmer until well reduced (depending on the passata, it could take 30 to 90 minutes). Meanwhile preheat the oven to 200°C.
For the light besciamella:
Heat the milk in a small casserole until it boils; remove it from the flame and slowly add the sieved flour. Stir until smooth, then add two tablespoons of olive oil and mix. Cook until creamy, then add salt, freshly ground nutmeg and pepper to taste.
Build the lasagna:
Shred the mozzarella. Put a few tablespoons of ragù and besciamella in an oven pan.
Add a layer of lasagna pasta, top with two tablespoons of besciamella, cover with ragù and add some mozzarella.
Repeat until you reach a monumental height (ours is around 7 layers of love).
Cook in the oven for 30-45 minutes, until the mozzarella on top gets slightly browned.