Place butter in a bowl and cook for 1 minute. Stir in beetroot (beets), vinegar, herbs and seasoning. Cover and cook for 8 minutes or until beetroots are tender.
Place cornflour (cornstarch) and milk in a small bowl and mix until smooth. Stir mixture into beets and cook, covered, for about 4 minutes until thickened.
Allow to stand, covered, for 2 minutes. Garnish with fresh dill or fennel.