Bring a large pot of water to a boil. Cook pasta until al dente according to package instructions, about 8 minutes.
Meanwhile, combine oil and pine nuts in a large saute pan. Cook over medium heat, stirring occasionally, until nuts are lightly toasted, 3 to 4 minutes.
Add onion, sliced fennel, garlic, and salt to pan; season with pepper. Cook, stirring occasionally, until onion is soft and light golden, 9 to 10 minutes. Add sardines; stir in lemon zest and juice. Chop reserved fennel fronds; stir into mixture.
Drain pasta, reserving about 1/4 cup cooking liquid. Add pasta to mixture in pan along with enough cooking water to coat; toss to combine. Divide among four serving plates; grate lemon zest over each. Serve immediately.