1: In a small stockpot, warm the vinegar over medium-high heat. After about 5 minutes, when the vinegar reaches 150 degrees F on an instant-read thermometer, just before it starts to simmer, add the lemon slices and continue to cook until the lemon peels begin to soften and wilt, about 10 minutes more.

2: Whisk in the black pepper, cayenne, pepper flakes, and sugar. Continue to cook over medium-high heat until the sugar is completely dissolved and the sauce reaches 190 degrees F, about 10 minutes. Remove from the heat and allow to completely cool before using. Once the lemon is removed, the sauce can be refrigerated in an airtight container for up to 8 weeks. Makes 1 gallon.