Roast and peel the beets. Let cool, then slice each into 8-10 segments.
In a medium frypan over medium heat, heat olive oil and cook onion rings with a pinch of salt, stirring frequently so as not to burn. Add the sugar after 10 minutes or so then cook until caramelized, another 10 to 15 minutes. Set aside on paper towels to drain.
Whisk together 3 tablespoons olive oil, vinegar and salt. Pour mixture in a re-sealable plastic bag, or, if using a mixture of beet varieties, divide among number of bags as different colored beets. Place beets in bag(s), seal, and shake gently to disperse dressing.
In a small bowl, mix Mascarpone and horseradish.
To plate, spread a tablespoon of horserasdish cream on a salad plate. Top with a quarter of the total beet slices, onions, one slice of cheese, a few pinches of parsley and a few cracks of ground pepper.