Preparations: Place a saucer in the freezer. Sterilize the jars.
Squeeze the lemons to obtain 500 ml/ 2 cups juice and the clementines to obtain 250 ml/ 1 cup juice. Strain the juice to remove the pulp.500 ml lemon juice/ 2 cups + 250 ml clementine juice/ 1 cup
Combine: Pour the juice into a large, wide pot. Add the sugar and stir well.750 g granulated sugar/ 3¾ cups
Bring to a boil, stirring well to dissolve the sugar. Once the mixture is boiling, set the timer to 10 minutes. Stir often while the jelly is cooking.
Start checking: After 10 minutes of cooking time start checking the jelly. Place a small amount of jelly on the freezer saucer and wait for about 30 seconds. If you are able to trace a line through the jelly, a line that stays that way, then the jelly is done. If not, return the pot to the burner and continue boiling for a few minutes more.
Try the consistency of the jelly again and continue in this manner until it is right. The time it takes to get the right consistency depends on the pot you are using. Mine was large and wide, which sped up the process; my jelly only needed 15 minutes.
Pour the jelly into the sterilized jars and close well.
Notes
Sub clementine juice with freshly squeezed orange juice.