Step 1

Bring a large pot of heavily salted water to a boil. Add pasta and cook until it is 1 minute shy of being al dente, usually a minute or 2 less than the package instructions. (It will continue to cook in the residual heat of the pot.) Scoop out about 1½ cups pasta water (a large coffee mug is good for this), then drain pasta.

Step 2

While the pasta is cooking, in a large skillet or Dutch oven, melt butter over medium heat. Cook, swirling occasionally, until the foam subsides, the milk solids turn golden-brown and it smells nutty and toasty, 3 to 4 minutes. (Watch carefully to see that it doesn't burn.)

Step 3

Stir in almonds, rosemary and red pepper flakes, and cook, stirring occasionally, until the nuts are toasted and slightly darker in color, about 1 minute.

Step 4

Add drained pasta, lemon juice and zest, salt and a large pinch of black pepper to skillet and about 1 cup of the pasta water. Add arugula and toss well to coat the pasta. Simmer to thicken the sauce until it's thick and glossy, adding more pasta water by the tablespoon as needed, about 1 minute longer.

Step 5

Serve topped with grated Parmesan cheese and more red pepper flakes, if you'd like.