Preheat oven to 400 degrees F.
Make the dressing first so that it can sit for a few minutes and let the flavors marry. Simply add mayo, mustards, 1 tbsp vinegar, honey and a pinch of sea salt/black pepper to a medium bowl. Whisk well to combine. Taste and season with more salt if needed. If you want it thinner, add the other tbsp of vinegar. Set this aside.
Line a large baking sheet with foil and spray with olive oil or coconut oil cooking spray, set aside.
Clean brussels sprouts and cut the ends off. Using a mandolin or sharp knife, shave the brussels into very thin slices. If using pre-shaved, omit this step.
Spread the shaved brussels onto the baking sheet and then place them in the oven for 4-6 minutes. Toss them and then place back in the oven for 4-6 minutes, until just browned/softened.
While the brussels are baking, toast the walnuts – place the walnuts into a small/medium dry/nonstick skillet over medium-low heat. Toss them every few minutes until fragrant and you start to see them get a little shiny/the oils start to release. Set aside.
Pull the brussels out of the oven and place the spinach on top and gently toss so that the spinach will begin to wilt from the heat of the brussels.
Add grated cheese, lemon zest, walnuts and dressing and gently toss to combine.
Season with sea salt and black pepper to taste. Serve immediately.