Put a large pan of salty water on the boil for the pasta. Break off the wooden ends of the asparagus and wash them thoroughly. Chop off the tips and reserve, slice the stems on the diagonal into about 1cm pieces.
Fillet the lemon – slice the top and bottom off the lemon. Run the knife along the sides from top to bottom, slicing off strips of the peel including the pith. Then cut out segments, running the knife in on the diagonal very close to the membrane - the segments should just drop from the fruit.
Put the pasta into the boiling water and cook according to the packet instructions. 3 minutes before the end of cooking time throw in the chopped asparagus. When ready, drain it into a colander, reserving a cupful of the cooking water.
Place a large pan or skillet over high heat. Add the butter, let it foam up, then add the asparagus tips. A minute later add the lemon pieces, turn up the heat and let the lemon dissolve. Mix in a third of the grated Parmesan.
Tip the drained pasta and asparagus into the pan, add some cooking water to make up the sauce, top with another third of Parmesan and toss it well in the sauce. Divide between plates and sprinkle with the remaining Parmesan.
