1. Wash oranges and wipe them dry. Cut each Seville orange in half, crosswise around the equator. Set a non-reactive mesh strainer over a bowl and squeeze the orange halves to remove the seeds, assisting with your fingers to remove any stubborn ones tucked deep within.
2. Tie the seeds up in cheesecloth or muslin very securely.
3. Cut each rind into 3 pieces and use a sharp chef’s knife to cut the rinds into slices or cubes as thin as possible. Each piece shouldn’t be too large (no more than a centimeter, or 1/3-inch in length.) Cut the navel orange into similar-sized pieces.
4. In a large (10-12 quart/liter) stockpot, add the orange slices, seed pouch, water, and salt, as well as the juice from the Seville oranges from step #1. Bring to a boil, then reduce to a simmer, and cook until the peels are translucent, about 20 to 30 minutes.
(At this point, sometimes I’ll remove it from the heat after cooking them and let the mixture stand overnight, to help the seeds release any additional pectin.)
5. Stir the sugar into the mixture and bring the mixture to a full boil again, then reduce heat to a gentle boil. Stir occasionally while cooking to make sure it does not burn on the bottom. Midway during cooking, remove the seed pouch and discard.
6. Continue cooking until it has reached the jelling point, about 220F degrees, if using a candy thermometer. To test the marmalade, turn off the heat and put a small amount on a plate that has been chilled in the freezer and briefly return it to the freezer. Check it in a few minutes; it should be slightly jelled and will wrinkle just a bit when you slide your finger through it. If not, continue to cook until it is.
7. Remove from heat, then stir in the Scotch (if using), and ladle the mixture into clean jars. Sometimes I bury a piece of vanilla bean in each jar. (Which is a great way to recycle previously-used or dried-out vanilla beans.)