Place potatoes in a medium saucepan; cover with water by 3 inches. Bring to a boil over high heat. Reduce heat to medium-low and cook until tender, 16 to 18 minutes.
Whisk together mustard, honey, vinegar, and tomato paste in a small bowl.
When potatoes have finished cooking, drain and transfer to a medium bowl. Add butter, za’atar, pepper, and salt; toss to coat. Drizzle with mustard mixture and sprinkle with onion and parsley.
