Step 1

In a food processor, combine red bell peppers and red pepper flakes. Pulse until very finely chopped.

Step 2

Combine bell pepper mixture, sugar, vinegar, butter, and salt in a heavy-bottomed pot. Bring to a vigorous boil over high heat. Boil mixture for 10 to 15 minutes (the longer it boils the thicker it will be - 10 minutes will give you a very loose jelly; 15 minutes will give you a more spreadable consistency). The jelly will thicken as it cools, so don’t worry if it seems loose right after boiling.

Step 3

Divide jelly between jars, leaving at least 1/3-inch of headspace at the top (this is especially important if you are planning to freeze the jelly because it will expand as it freezes).

Step 4

Let jelly come to room temperature and then add lids.

Step 5

Refrigerate or freeze jelly until ready to use. (Jelly will keep for several weeks in the fridge and up to six months in the freezer.)