Step 1

Heat ¼ cup oil in a large Dutch oven or other heavy pot over medium. Add walnuts and red pepper flakes and cook, stirring often, until nuts are deeply golden, about 5 minutes. Transfer to a small bowl and grate in whole garlic clove; set aside.

Step 2

Wipe out pot. Combine ¼ cup oil and remaining sliced garlic in pot and cook over medium-low heat, stirring occasionally, until oil around garlic is bubbling vigorously and garlic is beginning to brown, about 5 minutes. Firmly crush rosemary in your hand to bruise; add to pot along with beans and stir to coat. Increase heat to medium and cook, stirring occasionally, until beans are just beginning to take on color, about 3 minutes.

Step 3

Meanwhile, cook pasta in a large pot of boiling salted water, stirring occasionally, until very al dente, about 2 minutes less than package directions.

Step 4

When pasta is about 4 minutes away from being done, transfer 1 cup pasta cooking liquid to pot with beans (make sure to keep heat on medium so that the liquid doesn’t evaporate too quickly). Using a wooden spoon, gently mash about three-quarters of beans into a creamy sauce.

Step 5

Using a spider or slotted spoon, transfer pasta to pot with beans and add 1 cup pasta cooking liquid and 2 Tbsp. oil; mix well to combine. Increase heat to medium-high, bring to a simmer, and cook, stirring gently with a heatproof rubber spatula, until sauce coats pasta, about 2 minutes. (Sauce might look soupy but will thicken as it sits so don’t worry!) Finely grate in about one-quarter of zest from lemon, then cut lemon in half. Squeeze in juice from 1 lemon half and add parsley; mix well to combine.

Step 6

Divide pasta among bowls. Drizzle with oil, season with black pepper, and top with reserved nut mixture. Slice remaining lemon half into wedges and serve alongside for squeezing over.