Preheat the oven to 350 degrees F (180 degrees C).
Place the beets on a large sheet of foil. Season with salt and drizzle with 2 tablespoons (30ml) olive oil. Wrap the beets in the foil, leaving a little hole at the top, and place in a roasting pan. Cook until a knife inserted into the beets goes in easily, about 45 minutes.
When the beets are cool enough to handle, peel and chop into 1/2-inch pieces.
Whisk together the mustard, vinegar, hazelnut oil, maple syrup, tarragon, shallots and the remaining 1/3 cup (85ml) olive oil in a bowl.
Toss the beets with the dressing. Add the celery leaves and season with salt and pepper.