Process bread in a food processor to medium-sized crumbs. Sift using a coarse strainer to remove fine crumbs (discard). Heat 2 tbsp olive oil in a frying pan over medium-high heat, add breadcrumbs and fry until golden brown (4-6 minutes). Remove from pan and set aside.
Heat a large saucepan over high heat. Add mussels and white wine, cover and steam, shaking pan occasionally, until mussels just open (1-2 minutes). Drain, reserving 2 tbsp cooking juices, and set aside until cool enough to handle (1-2 minutes), then remove mussels from shells.
Cook pasta in a large saucepan of lightly salted boiling water until al dente (14-15 minutes), adding cavolo nero in the final minute.
Meanwhile, stir anchovies in remaining olive oil in a large frying pan over medium heat until anchovies dissolve (1-2 minutes). Add 'nduja and sauté, breaking it up, until collapsed (1-2 minutes). Add passata, stir for a minute, then add fish stock and bring to the boil. Drain pasta and cavolo nero, add to passata mixture and toss to release starch from pasta and thicken sauce (20-30 seconds). Add mussels and reserved cooking juices, toss to combine and season with black pepper and a squeeze of lemon. Transfer to plates, scatter with chives and fried crumbs, and serve.
