Start 3 cups of water to boil.Core and seed red peppers and cut into large chunks. Place in a food processor. Pulse until no large chunks remain. Remove to a large bowl and cover with boiling water. Let stand 15 minutes, then drain well.
Add drained red pepper puree to a large, heavy-bottomed saucepan. Add sugar, vinegar, salt and red pepper flakes, if using. Stir to combine. Heat over medium heat until mixture boils, then reduce heat to medium low and simmer, stirring regularly to start, then watching and stirring more regularly towards the end of cooking to avoid scorching on the bottom, until mixture is thickened, about 40-45 minutes. (Mixture will thicken further in the fridge).
Prepare three one-cup capacity jars by cleaning well.
When jam is ready, spoon into prepared jars and allow to cool 15 minutes uncovered. Place lids on jars, just finger tight and place in the refrigerate for 6-8 hours, before using.
Notes
This small-batch recipe is intended for short-term use and as it is not canned, it must be refrigerated after making. Can be doubled for a larger batch. For canning for longer term storage, please consult a safe-canning resource for guidance.Nutritional information is for the entire batch of jam.Be sure to read the Notes (above the recipe card!), where I share more detailed tips, variations and substitution suggestions for this recipe!