Remove thin outer rind from lemons with a vegetable peeler and cut into fine strips with scissors or a sharp knife, or use a zester.
Remove white rind in large pieces from lemons and place in large stainless steel saucepan with outer rind and add water. Boil over high heat, cover, reduce heat and boil gently for 20 minutes.
Finely chop lemon. Add lemon, pears, apples, and baking soda to saucepan. Bring to a boil over high heat, cover, reduce heat and boil gently for 20 minutes, stirring frequently.
Using tongs, remove and discard the large pieces of white rind.
Add sugar and ginger to saucepan. Return to a boil over high heat and boil rapidly, uncovered, until mixture will form a gel, about 20 minutes, stirring frequently.
Test for gel formation by dipping a cool metal spoon into the hot fruit mixture and immediately lifting the spoon so the mixture runs off. When mixture ‘sheets’ from the spoon (the drops become very thick and two drops run together before dropping off), it will form a gel on cooling and no further cooking is required.
Remove from heat. Laddle into sterilized jars and heat seal jars covered in boiling water for 10 minutes.