How to Make It
Preheat oven to 400°F. Bundle red beets together; wrap in aluminum foil. Bundle golden beets together; wrap in foil. Roast beets at 400°F until tender, about 1 hour. Unwrap beets, and let stand until cool. Peel and slice into 1/8-inch slices.
Roll dough round into a 14-inch circle on parchment paper dusted with whole-wheat flour. Carefully transfer dough and parchment to a baking sheet. Spread cheese over dough, leaving a 2-inch border. Beginning in center, arrange beet slices on cheese in overlapping concentric circles. Stir together vinegar and honey in a small bowl until smooth. Drizzle vinegar mixture evenly over beets. Sprinkle with salt, pepper, and thyme. Fold dough edges over beets toward the center. Brush egg over top of folded dough. Bake tart at 400°F until crust is deeply browned, about 30 minutes. Let stand 10 minutes. Slice into 12 wedges.