Bring 1 1/2 cups of water, a light sprinkling of salt, and the quinoa to boil over medium-high heat in a small covered saucepan. Reduce heat to low and continue to cook until water is absorbed, about 10 minutes. Turn off heat and let quinoa stand a few minutes longer. Spread quinoa onto a small baking sheet to cool.
Meanwhile, place Brussels Sprouts in a food processor; pulse to chop, 15 to 20 short bursts. Dump into a large bowl along with diced apple, red onion, dried cranberries, and walnuts (blitz them in the food processor).
Add vinegars, mustard, sugar (or maple syrup!), a light sprinkling of salt and several grinds of pepper to the food processor. Process to combine; With motor running, slowly add olive oil to make a smooth, emulsified dressing. Add to salad, toss, and serve.