If using fresh beets, peel and cook until tender, about 45 minutes at 375°F (190°C). Let cool and then slice or cube.

In a medium saucepan, combine apple cider vinegar, sugar, cornstarch, salt, and cinnamon. Cook over medium heat, whisking constantly until the mixture begins to thicken, about 3-5 minutes.

Once thickened, remove from heat and stir in the butter until melted and smooth.

Add the sliced or cubed beets into the saucepan and gently toss to coat the beets evenly with the sauce.

Simmer the coated beets in the sauce for 5-10 minutes, stirring occasionally.

Taste and adjust seasoning as needed, then serve warm.