Fill a large pot with water and add enough salt to make the water taste lightly seasoned. Set the pot over high heat and bring the water to a full boil.

While the sauce simmers, add the penne to the boiling water in the pot and cook until it is tender but still has a slight chew, usually 2 minutes less than the package instructions.

Place the walnut halves in a large, dry skillet over medium heat and toast them, shaking the pan occasionally, until they are fragrant and slightly browned, about 5 minutes.

Remove the walnuts from the skillet, let them cool slightly and chop them into small, coarse pieces.

In a small bowl, combine the chopped walnuts with the zest of the lemon and half of the torn basil; cut the remaining lemon into wedges and set aside for serving.

Add the olive oil, sliced garlic, and red pepper flakes to the same skillet over medium heat, cooking for about 2 minutes until the garlic is fragrant and golden.

Carefully pour the canned tomatoes and their juices into the skillet, using a wooden spoon to crush the tomatoes into smaller pieces, and let the sauce simmer gently for 10 minutes.

Before draining the pasta, scoop out 1 cup of the starchy cooking water and set it aside.

Drain the cooked pasta in a colander in the sink so it is ready to go into the sauce right away.

Add the drained pasta to the skillet with the tomato sauce, pouring in half of the reserved pasta water and the grated Parmesan cheese.

Toss everything together over medium heat until the sauce is glossy and coats the noodles, adding more pasta water if the sauce seems too thick and stir in the remaining basil and.

Serve topped with the walnut gremolata and lemon wedges.