Grate the 1/2 yellow onion into a glass mixing bowl.
Grate the garlic as well or use a garlic press.
Add in all these ingredients into the bowl: mustard powder, ginger, turmeric, cinnamon, allspice, sugar, and salt.
Drizzle in the honey.
Mix in hot water.
Pour and whisk everything together until well combined.
Stir into the apple cider vinegar.
Cover the bowl and let it stand at room temperature overnight.
After the mustard mix sits overnight, pour the mix into a blender and begin to puree, scraping down the sides occasionally. It took a few blending cycles to get the mustard thick.
If you are still having trouble getting your mustard to thickness you like, you can boil it down for a bit. Make sure to keep stirring the mixture so it doesn't stick. Keep on low heat after it starts to boil.