Bring a large pot of generously salted water to a boil, then add the pasta. Cook until just al dente, about 1-2 minutes less than package instructions. Drain and set aside, reserving a cup of starchy pasta water.
While the pasta cooks, place a wide skillet over medium heat and add butter along with olive oil. As it melts and combines, add the sliced leeks, spreading them out evenly in the pan.
Sauté the leeks slowly, stirring occasionally, until they turn golden and fragrant—about 8-10 minutes. They should smell sweet and develop a soft, caramelized appearance.
If using, add the minced garlic and cook for 30 seconds until fragrant, stirring constantly to prevent burning.
Gradually add the washed spinach in batches, stirring until each addition is wilted and vibrant green, about 1-2 minutes per batch.
Once the greens are wilted, add the drained pasta directly into the skillet with the greens. Toss everything together gently, adding a splash of the reserved pasta water to loosen the mixture if it seems dry.
Squeeze the juice and zest of a lemon over the pasta, then season with salt and pepper to taste. Toss again to combine all flavors evenly.
Finish by stirring in a small knob of butter for added richness, letting it melt into the pasta and greens for a glossy, flavorful finish.
Serve the pasta hot, garnished with extra lemon zest if desired, and enjoy the fragrant, vibrant flavors of this early-spring dish.
