Preheat oven to 400 degrees F. Divide beets between 2 pieces of foil; bring edges together and crimp to make packets. Roast until the beets are just tender when pierced with the point of a knife, about 1 1/4 hours. Unwrap the beets and let cool.
Meanwhile, whisk oil, vinegar, mustard, honey, salt and pepper in a small bowl to make dressing.
When the beets are cool enough to handle, slip off the skins. Cut into 1/2-inch cubes and place in a large bowl. Add celery, shallot and the dressing; toss to coat well. Serve at room temperature or chilled.
Make Ahead Tip: Cover and refrigerate for up to 2 days.
2 1/2 vegetable, 1 1/2 fat