Prep: Place a saucer with 5 metal teaspoons in a flat place in your freezer for testing the jam later. Rinse the mint well under cold water, pat it dry between two paper towels, and set aside.

Sterilize the jars: Gather five 8-ounce jars and a stock pot that will allow water to cover the jar tops by 1″. Wash the jars and set them in your pot, right-side-up. Fill pot with enough hot water to cover the jars by 1″. Bring to a full rolling boil over high heat (covering the pot with the lid will speed up the boiling) and then boil for 10 minutes, to sterilize the jars. Turn off heat and remove the jars using jar lifter tongs, drain well, and set aside to dry on a clean surface.

Clean the lids: Place jar lids in a small pan with enough water to just cover them. Bring to a slow simmer and keep hot until you fill the jars with jam. (Do not boil the lids, as the higher water temperature can harm the rubber sealing rings on the lids.)

Cook the jam: Combine the blueberries, sugar, and lemon juice in a wide nonreactive pan (do not use an aluminum pan) with an 11- or 12-quart capacity. Place the pan over medium-high heat and cook, stirring constantly, until the juice begins to run from the berries. When the juice starts flowing freely, increase the heat to high. Continue to cook, stirring very frequently, until the mixture boils. Once it reaches a boil, cook it for 10 to 15 minutes more, stirring frequently, and decreasing the heat slightly if the jam starts to stick. Begin testing for doneness after 10 minutes.

Test jam for doneness: To test for doneness, carefully transfer a scant half-teaspoonful of jam to one of your frozen spoons. Be careful to not add too much, as that will alter your doneness results – just a half of a teaspoon is all you want! Place the spoon back in the freezer for 3 minutes, then remove again and carefully feel the underside of the spoon. It should be neither warm nor cold. If it is still warm, return it to the freezer for a moment. Tilt the spoon vertically to see whether the jam runs. If it runs ever so very slightly, this is when I say it's ready. If it runs more freely, cook the jam for another minute, stirring, and test again as needed. If you like your jam very thick, wait until it doesn't run at all.

Skim jam & steep the mint: Turn off the heat and do not stir. Using a stainless steel spoon, skim any foam from the surface the jam. Place the mint sprigs into the jam and let steep for a minute or two off the heat. Taste carefully and either remove the mint or leave it in for another minute or two. Keep in mind that its flavor will be slightly milder once the jam has cooled. When the flavor is strong enough for you, use tongs to discard the mint.

Add jam to jars: Pour the jam into your sterilized jars, leaving 1/4″ headspace at the top - I like to use a ladle and wide-mouth funnel for filling the jars. Use a damp paper towel to wipe the rims of the jars clean, then put a hot flat lid and a ring on each jar, adjusting the ring so that it’s just finger-tight.

Return the filled jars to the stock pot, making sure the water covers the jars by at least 1″. Bring to a boil. I put the lid on the pot to speed up the boiling, then remove the lid once the boiling starts. Then boil for 10 minutes to process.

Remove the jars to a folded towel on the counter, with at least 1″ of space between jars. Do not disturb for 12 hours. As the jam cools, you may hear a few little pops as the lids seal. Before putting your jam away, be sure to feel the top of each lid to verify that it has sealed. It should be curving in very slightly in the middle and not be able to be pushed down any further. If any jars have not sealed, keep them in the refrigerator. Label the sealed jars and store for up to 1 year in a cool dark place. After you open a jar, keep it in the refrigerator.