Genoa and the Ligurian coast. The combination of trenette-pesto-potato-beans is the canonical serving format for Genovese pesto, documented from at least the 19th century.
Trenette — a long, flat Ligurian pasta similar to linguine but slightly thicker and with a more pronounced rectangular cross-section — served with pesto Genovese, waxy potato cubes, and French beans, all cooked in the same water. This is the full traditional recipe: the potato and beans are not optional additions but part of the dish's architecture. The potato starch and the beans create a body in the pasta water that emulsifies the pesto, creating a cohesive sauce rather than oily pasta.
Potato and pesto is a union: the fat in the pesto coats the starchy potato and creates a background sweetness that softens the raw garlic. The beans add vegetable freshness. Together the dish is rich but not heavy, with the basil and oil fragrance dominating.
Trenette is made with 00 flour and eggs — slightly stiffer than fettuccine. The traditional combination of potato (about 100g per serving, cut in 1.5cm dice) and French beans (topped and tailed, cut in thirds) cooks with the pasta because they require different times: potato goes in first with 3 minutes head start, beans after another 3 minutes, pasta last. Reserve 150ml pasta water before draining. Drain together and immediately toss with room-temperature pesto and 3-4 tablespoons of starchy pasta water. The tossing motion is key — it creates a creamy emulsion around the pasta.
Toss the drained pasta-potato-bean mixture in a warmed bowl with pesto and pasta water for 30 seconds — you should see a light, creamy sauce form. The pasta water is the emulsifying agent; don't skip it. Finish with a few torn basil leaves and a drizzle of pesto on top — visual and aromatic.
Adding hot cooked pasta directly to cold pesto out of the fridge — pesto chills on contact and becomes gluey. Not reserving enough pasta water — emulsification fails. Cooking potato and pasta separately and combining at the end — misses the starchy pasta water that has absorbed potato starch. Using store-bought pesto — the flavour profile of commercial pesto is entirely different and will dominate in a negative way.
Marcella Hazan, Essentials of Classic Italian Cooking; Slow Food Editore