1. Place a few small plates in the freezer. Stir berries, sugar, lemon juice, salt, and half the orange zest in a large, heavy pot. Bring to a boil, stirring to dissolve sugar and mashing lightly with a potato masher. Skim foam from surface. Cook, stirring more frequently as jam thickens, until it has the consistency of very loose jelly, 8 to 9 minutes. Remove from heat.

2. Remove a plate from freezer; drop a spoonful of jam on it. Return to freezer for 1 to 2 minutes; nudge edge of jam with a finger. It should hold its shape. If jam is too thin and spreads, return it to a boil, testing every minute, until jam holds its shape on plate.

3. Strain about half the jam through a fine sieve into a bowl; discard seeds. Return strained jam to pot; stir in remaining zest. Return to a boil, then remove from heat. Let cool before using or storing.