Heat oil in a saucepan over medium heat. Add onion and garlic and stir occasionally until tender (5-7 minutes). Add salami, cook until golden (2-3 minutes), then add fennel seeds and stir until fragrant (30 seconds). Add tomato and stock and simmer until slightly thickened (5-8 minutes).
Meanwhile, heat a deep-sided frying pan over high heat. Add mussels and wine, cover and stir occasionally until mussels open. Drain half the liquid, then add tomato mixture, bring to the simmer and season to taste.
For buttered polenta, bring stock to the boil in a large saucepan. Add polenta in a thin steady stream, whisking continuously, then bring to the simmer. Add milk, reduce heat to low and simmer until cooked (3-4 minutes; add water, if necessary, to achieve desired consistency), Add parmesan and butter and season to taste. Divide among plates, spoon mussels and cooking liquid over and serve.