Kosher salt and freshly ground black pepper,to taste
To make the dressing: In a a small bowl or jar, combine olive oil, lemon juice, honey, mustard, garlic, poppy seeds, salt, and pepper. Whisk or shake until well combined.
To make the salad: First, shave the brussels sprouts. Use a food processor with the slicing attachment and pulse until the brussels sprouts are thinly sliced. You can also use a mandoline or sharp knife if you don't have a food processor.
Place the brussels sprouts and spinach in a large bowl and toss. Add the blueberries, almonds, red onion, avocado, and feta cheese. Drizzle with the lemon poppy seed dressing and toss until the salad is well coated. Season with salt and pepper, to taste. Serve.
This salad is best the day it is made, but can be stored in the refrigerator for up to 2 days.