Cook millet according to package instructions.
Preheat the oven to 400 F.
Wrap the beets loosely in aluminum foil. Roast them until you can easily pierce them with a fork — about 50 to 60 minutes. Set aside to cool.
Heat the oil in a skillet over medium-high heat. Add the garlic and cook until it is fragrant, about 30 seconds. Add the onions. Cook on medium heat for 10 to 12 minutes, until the onions are dark and caramelized.
Pour in the cider vinegar and scrape up the caramelized bits, called the fond. Simmer until the liquid is gone and the pan is nearly dry again. Remove the pan from heat and set it aside to cool.
Split the black beans into two equal portions. Pulse half of them in a food processor. Place the other half in a large bowl.
Use the edge of a spoon or a paper towel to scrape the skins off the cooled roasted beets. The skins should slip off easily.
Grate the peeled beets on the largest holes of a box grater. Place the beet gratings in a strainer set over the sink. Press and squeeze the beet gratings to remove as much liquid as you can from the beets. You can do this over a bowl and save the beet juice for another purpose.
Preheat the oven to 350 F.
Place the squeezed beets, cooked millet, mushrooms and sauteed onions in the bowl with the beans. Sprinkle the smoked paprika, brown mustard, cumin, coriander and thyme over the top of the mixture. Mix all the ingredients until combined. Taste the mixture. Add pepper, or any other spices or flavorings as needed.
Finally, add the barley flour and chia gel.
Mix until you no longer see any flour. Form the mixture into 11 patties, about 6 ounces each.
Bake at 350 F for 15 minutes.
Serve the burgers on whole-wheat rolls.