Preheat the oven to 300°F.  Line several baking sheets with parchment paper.  Scrub the beets well with a veggie brush or scrubbing gloves.  Trim the top of each.

Using a mandoline, slice the beets paper-thin (1/16 inch). Hold the root end while dragging the beets across the mandoline, with caution.

Place the beet slices in a large bowl and pour the oil, rosemary & salt over the top.  Toss well.  (If using red and golden beets, place in separate bowls and divide the oil and salt evenly.)   Let the beets sit in the oil mixture until they release there natural juices, about 15-20 minutes.  This will allow them to retain a better shape and color.

Toss the beets again, then drain off the liquid.  Lay the slices out in a single layer on the prepared baking sheets.  Bake for 45-60 minutes until crisp, but not brown.  Test after 45 minutes and only bake longer if necessary.  Remove the beet chips from the oven and cool completely before storing in an air-tight container.