Rinse and rub a small thin-skinned lemon under plain running water, then dry. Quarter it while fresh. Remove visible seeds and trim the thick central white core. Put quarters in a covered freezer-safe container and freeze at least 4 hours or overnight, until hard throughout. Keep unused pieces frozen. Do not try to cut a whole solid frozen lemon with a knife.

Bring a large pan of water to a boil and salt as desired. Cook spaghetti until al dente according to its package, usually 9–12 minutes. Before draining, reserve at least 250 ml of the starchy cooking water. The recipe will use about 90–150 ml.

While pasta cooks, finely grate 20 g of frozen lemon flesh and yellow zest from a quarter into a small bowl. Avoid thick white rind and stop if you encounter a missed seed; do not grate it. Keep fingers clear of the grater and leave a small end ungrated. Grate up to 5 g extra separately for adjustment.

In a wide skillet over low heat, melt the butter with 60 ml hot pasta water. Add drained hot spaghetti and toss. Remove the skillet from the heat. Sprinkle in the Parmesan gradually while tossing, then add the 20 g grated lemon, parsley and black pepper. Add more hot cooking water a tablespoon at a time until a light glossy sauce coats the strands. Use about 90–150 ml water in total; avoid a hard boil after adding cheese.

Taste a strand before adding any salt or the remaining 5 g lemon. Divide among 3 warm bowls and serve immediately. Optional vegetables, proteins, bread or extra cheese are excluded from the estimates.

Refrigerate leftovers promptly in shallow covered containers at 40°F (4°C) or below, within 2 hours or 1 hour above 90°F (32°C). Use within 3 days. Reheat thoroughly to 165°F (74°C) with a splash of water; texture will differ from the fresh sauce.