Prepare water bath canner by filling to 3/4 full of cool water and adding jars. Bring to boil and turn down to keep at simmer. Combine pears and cranberries in a sturdy non-reactive stockpot or deep skillet. Cook over medium heat stirring frequently to prevent burning and to keep mixture evenly cooking. After 5-10 minutes, the cranberries will begin to pop. Add the sugar and lemon juice. Continue cooking until most of the cranberries have popped and juices have been released from the pears. Cook until mixture reaches the desired thickness â€“ about 20 minutes. You can test this by dropping a tsp. the mixture onto a saucer. Let cool for about a minute. Then pick up the saucer and tilt sideways. If the jam appears to keep itâ€™s shape, itâ€™s done. If it separates or runs down the saucer, cook a bit longer and test again. At this point, you may puree the mixture with an immersion blender or by spooning batches into a food processor or mixer. Or if you like the texture as is, you are ready to fill jars. Spoon into hot jars, leaving 1/2 inch headroom. Wipe rims, lid and place in canner. Bring water in canner back to a boil and process for 10 minutes. Remove to protected countertop. After 1 hour, test lids and place any unsealed jars in refrigerator. Let remainder cool for 12 hours. Label and date.