Heat the oven to 220°C. Rub the sausages with a little oil, put on an oven tray lined with greaseproof paper and bake for 25 minutes. Turn once halfway through to ensure they’re nicely browned all over.
In a small, dry frying pan toast the mustard seeds until fragrant. Core the apples, finely slice and cut into matchsticks. Peel the shallots and finely slice. Finely slice the fennel or use a mandolin.
Add to a bowl with the remaining olive oil, lemon juice, coriander, seasoning and mustard seeds. Stir to combine.
In a small bowl, mix together the mayonnaise and mustard.
Partially slice open the sub rolls and toast the insides. Spread each roll with a little of the mustard mayonnaise, top with the hot sausages and the slaw.