Prep the rapini: Bring a large pot of salted water to a boil. Trim the rapini ends and cut into 2–3 inch pieces, keeping leaves and florets.
Blanch and chill: Add rapini to the boiling water and cook for 1–2 minutes until bright green and just tender.
Transfer to an ice bath to stop cooking. Drain well and squeeze out excess water.
Sauté aromatics: In a skillet, warm 3 tablespoons olive oil over medium heat. Add sliced garlic and red pepper flakes.
Cook 30–60 seconds until fragrant, not browned.
Add rapini: Toss the drained rapini in the garlicky oil for 1–2 minutes to coat and soften slightly. Season with a pinch of salt and pepper. Remove from heat.
Blend the base: In a food processor, add the beans, lemon juice, lemon zest, and half the rapini mixture.
Pulse to begin combining.
Build the texture: With the machine running, drizzle in a bit more olive oil if needed and add 2 tablespoons water or broth to loosen. Blend until smooth and creamy, scraping the sides as needed.
Add the rest: Add the remaining rapini and the Parmesan (if using). Pulse several times until incorporated.
Keep it slightly textured or go fully smooth—your call.
Season to taste: Add salt and pepper, and more lemon juice if you want extra brightness. If it’s thick, blend in another tablespoon or two of water or broth.
Serve: Spoon into a shallow bowl. Drizzle with olive oil, add a pinch of red pepper flakes, and crack on fresh pepper.
Serve warm or at room temperature with toasty bread or crisp vegetables.