Preheat the oven to 400 degrees F.
In a bowl, toss the potatoes with 2 tablespoons of the coconut oil to coat. Sprinkle with salt and pepper, then spread the potatoes out in a single layer on a baking sheet. Toss the beets with the remaining 1 tablespoon coconut oil; sprinkle with salt and pepper and spread out in a single layer on a second baking sheet. Add 2 tablespoons water to the beets. Transfer the baking sheets to the oven and bake for 35 minutes, or until the potatoes are golden brown, stirring halfway through. If the beets start to look dry, add a little more water. Set aside to cool for 10 minutes.
Meanwhile, make the dressing: Add the mayonnaise to a bowl and whisk in the buttermilk and mustard. Set aside.
Transfer the cooked potatoes and beets (and any accumulated beet juices) to a salad bowl. Add the celery, green onions, dill, the dressing and some salt and pepper. Toss to coat well with dressing. Cover and refrigerate for 4 hours. Just before serving fold in the blue cheese.