Using a mandoline or very sharp knife, slice Brussels sprouts to an 1/8 inch thickness. They will be all different shapes and sizes, don't worry. This isn't about perfection.
Heat 2 Tbsp olive oil in a large saute pan over medium-low heat.
Once oil is warm, add diced onion and minced garlic with a pinch of salt.
Cook, stirring, for 2-3 minutes until softened.
Add in the sliced Brussels sprouts along with 1/2 tsp of Kosher salt and toss to coat.
Add in the lemon zest and juice and cook, stirring, for about 3 minutes until just softened.
Lastly, add in poppy seeds and cook for 30 seconds before removing from the heat.
Taste for seasoning and add 1/2 tsp salt if desired.
Top the whole pan with parsley and spoon into a large platter.