1.Trim fennel; reserve fronds. Using a V-slicer, mandoline, or a very sharp knife, slice the fennel as thinly as possible; combine with lemon juice in a small bowl.
2.Heat oil in a large saucepan; cook onion, stirring, until soft. Add garlic; cook, stirring, for 1 minute.
3.Stir rind, wine, chilli and saffron into onion mixture; cook, stirring, for 2 minutes. Add tomatoes; simmer, uncovered, for about 10 minutes or until mixture thickens slightly.
4.Add stock; simmer, uncovered, for about 20 minutes or until liquid is reduced by about a quarter.
5.Add marinara mix to tomato mixture. Cover; simmer, stirring occasionally, for about 5 minutes or until prawns change colour and mussels open.
6.Serve stew topped with fennel mixture; sprinkle with reserved fronds and rind, serve with crusty bread.
Some mussels might not open, these may need to be opened with a knife, or might not have cooked as quickly as the others. Farmed mussels will not all open up during cooking, and some will not open after excessive cooking, you do not have to discard these, just open with a knife and cook a little more, if you wish.