Prepare sauce by combining tahini, vinegar, date syrup, chili garlic sauce, and curry powder in a bowl. Gradually whisk in water in 1 Tbsp increments until sauce is smooth, and desired consistency is achieved. Start by seasoning with 1/4 tsp salt, adding up to 1/2 tsp to taste.
Store covered in the refrigerator for up to 10 days, or use on grilled shrimp skewers below:
Thread shrimp, red onion, and lemon slices onto 4 skewers. Season with salt and pepper to taste, and lightly coat with cooking spray.
Prepare a grill or grill pan for medium-high heat; clean grates well, then oil. Grill shrimp, turning once, until cooked through and lightly charred, about 5 minutes total. Serve with tahini sauce.