Toss cauliflower and sweet potato with olive oil, salt, and pepper then spread out in an even layer on a foil-lined baking sheet. Roast for 20-25 minutes or until browned.
In a small bowl, whisk together tahini, lemon juice/zest, garlic, and salt. Slowly add in the hot water until desired consistency is reached. Stir in parsley and set aside.
When vegetables are done roasting, serve on a platter and drizzle with tahini lemon parsley sauce. Serve warm or at room temp.