Remove the seeds from the peppers and place the peppers into a blender or food processor. Process until smooth. Pour into a large pot and stir in the mustard, sugars, apple cider vinegar and salt. Bring to a boil, so that it is boiling so hard it cannot be stirred down.
Stir together the flour and cold water until smooth. Make sure your water is cold - and make sure it’s mixed completely or your mustard will clump! Now, if you wind up with clumpy mustard.. it will still taste good so don’t throw it out. I like to use a mason jar with a lid and I shake, shake, shake until it’s all mixed and smooth.
Pour into the boiling mixture. Continue to boil, stirring constantly, for 5 minutes. Pour into sterile pint jars and seal with new lids and rings. Process in a boiling water bath for 5 to 10 minutes, depending on your altitude.