Make Ahead: You can choose to make the vegetarian haggis the day before, but put it in the pudding basin and steam it just before you want to serve it. Leftovers: Any leftovers should be covered and placed in the fridge for up to 4 days. Freeze: Place in an airtight freezer container or zip top bag for up to one month. Reheat: Thaw in the fridge overnight. Reheat the vegetarian haggis in a saucepan on medium heat until heated through. Taste and adjust seasonings.