1. Preheat oven to 400 degrees F. Wash the beets and sprinkle with the salt. Wrap each beet tightly in a foil. Place beets onto the center of the middle rack of the oven and roast until tender (60-90 minutes). Unwrap beets and allow cool enough to handle safely (15-20 minutes).
2. While the beets are cooling, heat the olive oil in a small skillet over medium-high heat until shimmering. Saute the minced onion until soft and starting to brown (5-6 minutes).
3. Remove onions from heat and whisk in the mustard and vinegar.
4. Using paper towels, wipe away the skins of the beets.
5. Divide spinach, beets, onions, and vinegar mix evenly. Top each plate with pistachios, cheese, and scallions.