Trim off the bottoms and outer leaves of your Brussels sprouts and then shave or slice them really thinly. The most efficient way to get this done is by using the slicing disk of your food processor, but alternatively, you could grate them with a box grater or slice them thinly with a sharp knife. Shave/shred the red cabbage in the same fashion.
Combine the shaved Brussels sprouts, leafy greens, shredded red cabbage, diced apple, chopped dates, pumpkin seeds and crumbled feta in a large mixing bowl. Set aside.
In a glass measuring cup, combine all the ingredients for the dressing and whisk vigorously until well combined and slightly emulsified. Pour over the reserved salad and toss gently, preferably with your hands (they do a much better job, trust me) until all the ingredients are well coated in the dresssing.
Allow the salad to sit for a couple of hours so the flavors have a chance to meld and fully develop.
This salad will keep in the refrigerator for up to a few days, although some discoloration is to be expected.