Start by washing the tomatoes and cutting a small X at the bottom of each one. This will make them easier to peel.
Bring a large pot of water to boil. Drop the tomatoes in for about 30 seconds to a minute, then immediately transfer them to a bowl of ice water. This process, known as blanching, will allow you to easily peel the skins off.
Once cooled, peel the tomatoes and chop them into small pieces. If the kids are helping, this could be a good exercise in practicing knife skills under supervision.
Put the chopped tomatoes in a large, non-reactive pot. Add the sugar, lemon juice, ginger, cinnamon, cloves, sea salt, and chili flakes if you're using them. Stir everything together.
Bring the mixture to a boil over medium-high heat, then reduce to a simmer. Let the mixture simmer for 1.5 to 2 hours, or until it thickens to a jam-like consistency. Stir occasionally to prevent the jam from sticking to the bottom of the pot.
While the jam is cooking, sterilize your jam jars and lids. This can be done in a dishwasher or by boiling them in a large pot of water for at least 10 minutes.
Once the jam is ready, carefully spoon it into the sterilized jars, leaving about a 1/4 inch of space at the top. Wipe the rims clean, then screw on the lids.
Now, it's time to process the jars to ensure they're safe for storage. Place the jars in a large pot (use a rack or a folded kitchen towel at the bottom to prevent the jars from touching the pot directly), fill it with water until the jars are covered by 1-2 inches of water, then bring to a boil. Let the jars boil for 10 minutes, then carefully remove them and let them cool.
Check that the lids have sealed properly by pressing down on the center of each one. If it doesn't pop back, it's sealed. If it does, refrigerate that jar and use it first.
Store the sealed jars in a cool, dark place. They should keep for up to a year. Once opened, store in the refrigerator.