Steam or boil potatoes until just fork tender. Meanwhile, line a skillet with a thin layer of water and cook onions until soft and translucent. Once onions are cooked, sprinkle dry mustard over top, stirring to incorporate, and set aside.
Whisk vinegar, water, agave nectar, flour, nutritional yeast and Dijon mustard together in a measuring cup.
Add 1/2 to 1 tsp salt and black pepper, or to taste, and set aside. (You can also add a little more agave if you don't want a strong vinegar flavor.) Once potatoes are cooked, run under cool water for a few seconds so they are safe to handle, but still warm. Gently remove skins. Cube potatoes; you should have approximately 3 cups of cubed potatoes. In a mixing bowl, toss potatoes with onion mixture. Re-whisk vinegar mixture, and pour over potatoes, stirring to combine.
Add the majority of the bacon bits, reserving about 1 tbsp to sprinkle over top, and mix again.
Let the mixture rest for 10 to 20 minutes, if possible. Then, gently mix in chopped scallions.
Transfer to a serving bowl and garnish with remaining bacon bits and more scallions.
