Preheat oven to 400 degrees F.
Arrange the vegetables on a large baking sheet or sheet pan (lined with parchment paper for easy cleanup), trying not to overcrowd the pan (thus allowing them to get crispy rather than soggy). Drizzle with olive oil and salt; toss to coat. Roast the vegetables for approximately 20-25 minutes, or until golden brown and fully cooked.
As the Brussels sprouts and sweet potatoes cook, prepare the tahini sauce by whisking together all the ingredients in a medium bowl. Season with salt to taste and add water as needed to reach desired consistency.
Remove the vegetables from the oven and drizzle (or dip) with the lemon tahini sauce. Garnish with additional salt, pepper, and chopped fresh parsley, if desired.