Step 1

Bring a large pot of water to boiling. Remove stems from fennel bulbs; trim and coarsely chop the fronds. Halve bulbs, then cut into ¼-inch-thick slices.

Step 2

Add penne and onion to boiling water. Cook according to package directions for the pasta, adding squash and sliced fennel bulb the last 7 minutes of cooking.

Step 3

Reserve 3 cups cooking water. Drain pasta and vegetables. Return pasta and vegetables to pot. Add 1 to 1½ cups cooking water and ¼ to ½ cup chopped fennel fronds to pot; toss to coat. Add additional reserved cooking water if desired for saucier pasta. Sprinkle with walnuts.