Step 1 Bring a large saucepan of water to the boil. Cook pasta according to packet instructions, or until tender. Drain, reserving ½ cup pasta water. Return pasta to pan.
Step 2 Meanwhile, bring a separate large saucepan of water to the boil. Cook Brussels sprouts for 5 minutes, or until just tender; drain, slice and add to pasta.
Step 3 Drain olive oil from anchovies into a medium frying pan set over medium–low heat. Chop anchovies into fine strips and add to pan with onion and garlic; cook, stirring, for 5 minutes, or until soft. Add capers, lemon zest and juice. Cook for 1 minute, or until mixture is heated through.
Step 4 Add reserved pasta water and anchovy mixture to pasta. Toss to combine and serve.