In a food processor, pulse pears and apples until fruit is chopped into very small pieces. Transfer the fruit to a large Dutch oven or other heavy-bottomed pot. Stir in lemon juice, cinnamon, allspice, ginger, cloves, nutmeg, pectin and butter.
Bring to a rolling boil over medium-high heat. Add sugar and stir to mix well. Return to a rolling boil, then boil for exactly 1 minute. Remove from heat; skim off foam with a metal spoon.
Ladle jam into hot sterilized jars, leaving ¼-inch headspace. Wipe rims clean with a damp cloth; add lids.
Process in a boiling-water bath for 10 minutes. Transfer jars to a towel and cool completely. Check for seals and refrigerate any jam that did not seal properly.
Notes
Leave about ¼-inch of headspace in each jar. Leaving the proper amount of headspace in a jar is important to ensure a vacuum seal. If there’s too little headspace, the jam may expand and bubble out when air is being forced out from under the lid during processing.
The jam will not set immediately, so it’s important to allow the jars to rest without interruption at room temperature until cool (or overnight).
Overcooked Pectin: It’s important to boil the jam with the sugar for exactly 1 minute. If you overcook the pectin, it may break down and fail to gel.
Add More Spices: The seasoning is very subtle in this jam. If you like a stronger ginger pear jam, add more spices to taste.
Reduced Sugar: I know that 5½ cups sounds like a lot of sugar, but trust me: you need all of it! Stick with regular granulated sugar (instead of sugar substitutes like Splenda or stevia) and do not decrease the quantity called for in the recipe. Jam making is an exact science, so measuring the correct quantities of fruit, sugar and pectin are essential to a successful batch of jam.