First, let's make our flavorful dressing! Add all the dressing ingredients (that's the olive oil, apple cider vinegar, sugar-free maple syrup, Dijon mustard, dried thyme, grated garlic, salt, and pepper) into a mini chopper, immersion blender, or food processor.
Blend until everything is super smooth and well combined – it should look creamy and emulsified. Set this delicious dressing aside for a moment.
Now, for the Brussels sprouts: give them a good wash, pat them dry, and gently peel off any loose outer leaves. The key to a great salad here is to shave them as thinly as possible! The easiest way to do this is with a mandolin slicer. If you don't have one, a very sharp knife or a food processor with a slicing blade will also work perfectly – just ensure they are truly shaved, not just coarsely shredded.
Transfer your thinly shaved Brussels sprouts to a large mixing bowl.
Into the same bowl, add the vibrant thinly sliced red cabbage, the crunchy toasted walnuts, the crumbled feta cheese, and the sweet fresh blueberries.
Pour your prepared dressing generously over all the ingredients in the bowl.
Toss everything gently until every bit is beautifully coated with the dressing. For the best flavor, let the salad sit for at least 20 minutes before serving – this allows the flavors to truly meld together.