1.Trim spring onions, leaving about 6cm of the stem. Slice thinly.
2.Melt butter in large frying pan, cook onion and fennel, stirring, until onion softens.
3.Add flour to frying pan, cook, stirring, 1 minute. Gradually stir in stock and cream, cook, stirring, until sauce boils and thickens. Stir in fennel tops, mustard, juice and peas. Add fish, stir to combine, remove from heat.
4.Preheat oven to 200°C (180°C fan-forced).
5.Spoon mixture into four 1½-cup (375ml) ovenproof dishes, place on oven tray.
6.Cut four 2cm-wide strips from pastry. Cut four pastry lids large enough to fit the tops of the dishes. Brush dish edges with egg, place pastry strips around edge of dishes, top with pastry lids. Brush with a little more egg.
7.Bake pies about 35 minutes or until pastry is puffed and browned.