Preheat the oven to 200C/400F/Gas 6.
Place the haddock on a baking tray and drizzle with the oil. Bake in the oven for 4 minutes or until cooked through.
For the peas, boil the chicken stock with the garlic clove for 5 minutes, then add the peas to cook through.
Add the cream. Using a mini hand-blender, purée the stock and cream and set aside.
For the sauce, melt the butter in a pan, then add the cream, lemon juice and mustard. Simmer over a medium heat until reduced by half.
Stir through the chives and season with salt and freshly ground black pepper.
To serve, place the haddock on a plate with the peas to one side. Drizzle the sauce on the fish and garnish with the chervil.