Preheat oven to 400 degrees F (204 C).

Wash beets with water and dry with a paper towel. Wrap each beet in aluminum foil and roast on a pan in the oven for 1 to 1 and a half hours. You'll know the beets are done when a knife slips in and out of one very easily. 

Allow the roasted beets to cool for at least 30 minutes and then slice them into even, thin slices. This is a good opportunity to use a mandoline for slicing if you have one. 

In a small pot over medium high heat, bring cider vinegar, water, sugar, salt, and ground mustard to a boil in a medium pot and stir/simmer until sugar dissolves

Add 5 peppercorns and the sliced beets to a jar. Pour the hot vinegar mixture over the top of the beets in the jar. 

Allow the jar of beets to cool to room temperature for about an hour. Add jar full of beets to the refrigerator and store for up to a month.