Put the beets in a large pot of water, season heavily with salt and bring to a boil over medium-high heat. Reduce the heat to medium and cook until tender, 20 to 25 minutes. Remove the beets from the water, cover with a clean kitchen towel and allow to cool for 5 minutes.
Meanwhile, combine the lemon zest and juice, parsley, olive oil, thyme and green onions in a medium bowl. Season with salt and pepper.
Using the same kitchen towel, remove the skins of the beets. Cut the beets into 1/2-inch-thick half-moon pieces and add to the dressing. Cover with plastic wrap and refrigerate for 1 hour.
When ready to serve, add the marinated beets to each individual plate. Top with some crumbled blue cheese and watercress.