Boil or roast the beets until you can pierce them with a fork. Boiling them takes about 40-50 minutes for medium size beets. Bigger beets will take longer to boil. If you are roasting them, set the oven to 400 degrees F (about 200 C). Let beets cool, then peel when cool enough to handle by hand. Store them in an airtight container in the refrigerator. Slice when it's time to add to salad.
I used the already roasted chestnuts for this salad, about 7 ozs (200 grams) which was 2 packaged bags. If you're roasting them yourself, cut a cross shape the round side of chestnuts, add them to a baking dish and roast at 400 degrees for about 15 minutes. Let cool them peel them. The skin will crack as it has gotten drier from roasting so they're easy to peel.
Prepare the salad platter by adding the spring mix greens first. Then slice the beets and spread them out on the platter and finally add the chestnuts. Usually I let the onion sit in a container with cold water for few minutes before slicing the onion. Slice a quarter of white onion very thin over the salad.
Dressing
Add in a jar olive oil, balsamic vinegar, dijon mustard, honey, salt and pepper. Close the jar and shake until all ingredients mix well.
Pour over salad right before serving and toss it all together or you can serve salad in a platter for everyone to individually serve on their plate, then each person adds dressing on their plate as desired. If I don't toss the salad myself, I usually double on the dressing to make sure I have enough for everyone.
Notes
If you can't find spring mix, arugula is a great alternative.
For a milder dressing replace balsamic vinegar with white wine vinegar.