If using a mix of colors, separate the red beet(s) from the golden and/or Chiogga beets. Using a knife or a mandolin, thinly slice the lighter-hued beets; add to a medium bowl.
Thinly slice the red beets; add to a separate medium bowl. Whisk vinegar, oil, and 3/4 teaspoon salt in a small bowl. Divide dressing between the separated beets; toss to coat. (If you’re not concerned about colors bleeding, or all of your beets have the same shade, no need to separate; whisk dressing in the bottom of a medium bowl, add all of the beets and toss to coat.)
Mix mascarpone (or sour cream) with remaining 1/4 teaspoon salt. Thin with cold water, 1 teaspoon at a time, until drizzly yet thick, like melted ice cream.
Layer beets on a serving platter. Pour any remaining dressing from the bowl that held the lighter beets over the top. Drizzle with thinned cream. Scatter with chopped hazelnuts, herbs, and a few grinds of black pepper to taste.