Bring a large pot of well-salted water to a boil. Cook farfalle until al dente, 1 minute less than package directions. Reserve 1/2 cup pasta water before draining. Drain and spread pasta on a sheet pan or wide bowl to cool slightly. Do not rinse.
Whisk olive oil, lemon juice, lemon zest, Dijon, garlic, honey, salt, pepper, and red pepper flakes together until smooth and emulsified. Taste and adjust seasoning — it should taste assertively bright before it goes on the pasta.
Pour two-thirds of the dressing over the still-warm pasta and toss to coat. Let sit 5 minutes. Add arugula and toss again — the residual heat will soften the leaves slightly without fully wilting them.
Add red onion, pistachios, and parmesan. Drizzle remaining dressing over the top and toss to combine. Taste and adjust salt and lemon. Add a splash of reserved pasta water if the salad looks dry.
Transfer to a serving bowl. Top with extra parmesan and a few fresh arugula leaves. Serve warm, at room temperature, or cold.
