To prepare the limes, slice off the ends to discard them, then quarter the fruit and thinly slice. Transfer the sliced fruit to a big pot and add the cold water. Put the lid on the pot and let the sliced fruit soak for about 12 hours (overnight).
After soaking for 12 hours, uncover the pot and place it on the stove. Turn the burner on to high heat and bring the mixture to a boil. Once at a boil, cover the pot (or partially cover) and reduce the heat to a slow simmer (medium–low or even low) and let the fruit slices simmer for 1.5 hours.
Uncover the pot, and let the citrus continue to simmer for another 30 minutes. The water will slowly evaporate over this time and most importantly, the white pith of the lime peel will become completely translucent. The fruit slices will be very, very soft and tender. This Cuisinart pot from Amazon is a decent size for the job. The goal is to soften the pith (the white part of the peel) so that it is completely translucent. During this step, most of the water will evaporate and you will be left with about ½–⅓ the original volume. Do not try to speed up this step and do not boil the fruit too vigorously or else you risk the peel being chewy and hard!
Once the fruit slices are very soft and the volume has reduced by almost two-thirds, turn off the heat and let the fruit sit for one hour before proceeding. This is to further ensure that all the fruit has softened sufficiently.
Add the sugar and lemon juice to the pot with the softened citrus fruit. Bring the mixture back up to a boil over medium–high heat, and continue to cook, stirring occasionally with a wooden spoon or heatproof spatula (grab this spatula from Amazon), until you reach the marmalade setting point (104–105 ºC or 220 ºF)—use a fast-reading digital thermometer like this one from Thermoworks. You can also check the consistency by letting the marmalade drip off a spatula over the pot. It should fall in a thick sheet as opposed to a very fluid, loose droplets.
When the marmalade reaches the set temperature, slide the pan off the heat, and let it cool for 2 minutes, then give it a good stir. This is to ensure that your canned marmalade will have an even dispersion of peel/fruit.
Line a rimmed baking sheet with a large kitchen towel (these Nordic Ware sheet pans from Amazon are great). This will be the “cooling station” for the jars.
Divide the mixture between 5 or 6 sterilized jars (I prefer wide-mouth mason jars like these Ball jars on Amazon), leaving a headspace of ¼ inch. If the last jar has a larger headspace, that will be your jar to enjoy right away, storing it in the fridge.
Wipe the rims of all the jars with a lightly moistened paper towel. Top each of the jars with a sterilized lid, and tighten the band until it is finger tight. The Ball canning kit on Amazon is key for this step and the next! Get it on Amazon.
Flip the closed jars upside down and let them stand, inverted, for 2 minutes, before flipping back upright. You will hear a popping sound soon after you've flipped the jars, a good sign that the vacuum seal is created.
Let the jars cool, untouched, for 24 hours before putting them away.
Notes
This recipe makes 5 to 6 jars of 250 mL of lime marmalade. Make sure to prepare 6 jars to make sure you have enough ready when it's time.To prepare jars for this recipe, start with washed jars. Set them on a sheet pan in an oven at 250 ºF, and leave them for at least 30 minutes before filling them.Note that lime marmalade isn’t green, and more of a golden colour. If you want the marmalade to be green, you will likely have to add a drop of food colouring.