Fillet steak sandwich with caramelised red onions and homemade stout mustard

Ingredients For the mustard 50g/2oz white mustard seeds 50g/2oz black mustard seeds 75g/3oz soft light brown sugar 2 tsp ground allspice pinch ground cinnamon 1 tsp coarse salt 1 tsp black peppercorns 1 tsp paprika …