To make the sauce:Whisk together the mustard sauce ingredients in a small bowl. Cover and refrigerate until ready to use. Bring to room temp before using.
To make the slaw:
Pulse the green onions, vinegar, water, chiles, mayo, and oil in a food processor or blender until smooth. Season with salt and pepper.
In a large bowl, toss the cabbage and red onion with dressing. Fold in the cilantro. Season with salt and pepper as needed. Let come to room temp for 15 minutes before using.
Preheat grill to medium heat. Clean and prep grill grate. Divide the beef into 4 equal portions. Press into patties and using your thumb, make an indention in the center of each.