Simmer leeks and spring onions in vegetable stock with a bay leaf for 10 minutes until tender.
Remove vegetables and boil the stock until reduced by two thirds.
Soak gelatine in cold water, strain, and whisk into the hot reduced stock.
Strain the stock into a bowl, season, stir in chervil, and chill for 30 minutes until partially set.
Layer leeks and spring onions on three sheets of cling film.
Spoon the jelly over the vegetables, roll into a tight cylinder, and chill for one hour.
Whisk mustard, vinegar, lemon juice, and oil together to create a dressing.
Slice the terrine into wedges and serve with ham slices and mustard dressing.