1½ lbs red, orange, and yellow sweet peppers, very roughly chopped
2 teaspoons dried hot red-pepper flakes
3 tablespoons Sure-Jell less- or no-sugar-needed pectin (from a 1¾-oz box)
3¼ cups sugar
1 cup white-wine vinegar
1 tablespoon unsalted butter
¾ teaspoon salt
Special equipment:
4 (1/2 pint) canning jars with screw bands and lids
Candy thermometer
Canning tongs
Instructions
Sterilize jars and lids: Wash jars, lids, and screw bands in hot, soapy water. Rinse well, then dry screw bands. Put empty jars on a rack in a boiling-water canner or a deep 8 quart pot and add enough hot water to cover by 2 inches. Bring to a boil, covered, then boil 10 minutes. Remove from heat, leaving jars in water, covered.
Meanwhile, in a small saucepan, heat lids and enough water to cover them by 2 inches. Do not boil the water, but heat until a thermometer reads 180°F. Remove from heat. Keep jars and lids submerged in hot water, covered, until ready to use.
Make jelly: Pulse bell peppers with red-pepper flakes in a food processor until finely chopped.
Whisk together pectin and ¼ cup sugar in a small bowl.
Stir together pepper mixture, vinegar, butter, salt, and remaining 3 cups sugar in large heavy pot. Bring to a vigorous boil over high heat, then continue to boil vigorously, stirring occasionally, 5 minutes. Gradually add pectin mixture, whisking constantly. Return jelly to a vigorous boil, stirring constantly, and boil, stirring constantly, for 2 minutes. Remove from heat.
Carefully remove jars and lids with canning tongs, then drain jars upside down on a clean kitchen towel and quickly dry lids. Invert jars and immediately ladle hot jelly into jars, leaving ¼ inch of space at top. Run a clean plastic spatula between jelly and sides of jars to eliminate air bubbles. Wipe off rims of filled jars with a damp clean kitchen towel, then firmly screw on lids with screw bands. If you have any extra jelly, you can just put it in an extra jar and store in the fridge to use up first (you won't need to process it).
Seal and process jars: Put sealed jars on rack in canner or large pot and add enough hot water to cover by 2 inches. Bring to a full boil, covered, then boil jelly, covered, 15 minutes. Transfer jars with canning tongs to a towel-lined surface to cool and to seal. Jelly will thicken as it cools.
After jars have cooled, about 12-24 hours, press center of each lid to make sure it is concave. Remove the screw bands and try to lift the lid with your fingernail. If you can't, the seal is good and can be kept, unrefrigerated, for about 6 months. Put any jars that haven't sealed properly in the refrigerator and use them first.