In a food processor, puree the peppers, garlic, mustard and vinegar.
Pour the puree into a nonreactive saucepan, then add the sugar and bring it to a boil over high heat. Lower the heat and simmer the mixture for 30 minutes.
In a small bowl or juice glass, mix the flour and 1/2 cup water to make a smooth paste. Whisk it into the pepper mixture and continue to simmer for 20 minutes, stirring regularly, until it becomes very thick. Let the sauce cool, then pour it into a covered nonreactive container (such as a glass jar). The sauce can be refrigerated for up to 1 month.
Photograph by Andrew Mccaul
Recipe courtesy Michael Symon for Food Network Magazine