1. Drain the tofu; slice it 1/2-inch thick and place the slices on a dish towel or several layers of paper towel. Cover with another cloth or layer of paper towels and place a cutting board on top weighted with a few heavy cans or a pot of water. Press the tofu slices for 1 hour, then cut into 1/2-inch cubes. You can also use baked, pressed tofu, in which case you just need to cut it into cubes.
2. Wash the beet greens thoroughly, drain, remove stems and shred.
3. Heat the canola oil in a skillet and add the onion. Sauté over medium-high heat until onion is translucent, then add the tofu and continue cooking until tofu begins to color.
4. Add the beet greens and garlic. Stir-fry until greens are cooked, about 5 minutes.
Season with the soy sauce or teriyaki sauce and cook for another minute to blend flavors. Serve over rice.