Dissolve cornflour in 1/4 cup (65ml) water and set aside.
In a medium saucepan over medium heat, bring chicken stock to a boil. Add Brussels sprouts and cook until tender - about 10 minutes. Strain, reserving chicken stock and place Brussels sprouts in a warm serving dish.
Return chicken stock to stove, stir in mustard and lemon juice and return to boil. Add cornflour mixture. Cook and stir until thickened. Pour over Brussels sprouts to serve.