This was blisteringly, slap you up side of your head, sinus clearing when I first made it. After a couple of hours, it calmed down to where it was still spicy, but you could also taste the mustard instead of getting just heat. It's a very small batch as I wanted to try the twist, which is the addition of onion powder and honey, plus substituting rice vinegar for part of the liquid instead of just water.
I did increase the amount of mustard powder slightly in the recipe below because the mustard was a bit on the thin side, more like a medium thick sauce than mustard.
2-1/2 Tbsp mustard powder (preferably Colman's)
1/4 tsp onion powder
1/4 tsp salt
1 Tbsp very hot water
1 Tbsp rice vinegar
1/2 tsp honey
Mix the mustard powder, onion powder and salt in a small bowl. Add the hot water and stir until mixed well. Add the rice vinegar and stir until mixed well. Stir in the honey. Let stand at least 15 minutes up to 2 hours. The heat will mellow the longer it stands.