Step 1 Heat olive oil in a heavy-based saucepan (which has a tight-fitting lid) over medium–low heat. Add leek and fennel; cook, stirring, for 6–7 minutes, or until vegetables soften. Add garlic to pan; cook, stirring, for 1 more minute.
Step 2 Increase heat to high and add white wine to saucepan; simmer for 2 minutes. Add stock, mussels and cannellini beans. Cover pan tightly and simmer, shaking pan occasionally, for 6–7 minutes, or until mussels open. (Discard any unopened mussels.)
Step 3 Meanwhile, toss rocket and roast capsicum in a salad bowl.
Step 4 Remove saucepan from heat. Season mussels and broth with cracked black pepper, then ladle into 4 bowls. Garnish each bowl with fennel fronds and serve with a slice of sourdough and salad.