Rinse fresh raspberries gently and let drain. If using frozen, no need to thaw—add them directly to the pan.
In a medium nonreactive saucepan, combine raspberries, sugar, and lemon juice. Stir gently to mix.
Bring mixture to medium heat. As it warms, press berries with a spoon or masher to release juices. Maintain a gentle boil, stirring occasionally so it doesn’t stick.
Simmer 12–20 minutes until thickened. Test by placing a small spoonful on a chilled plate—if it wrinkles when you push it, the jam is set.
Remove from heat, skim foam, stir in butter and lemon zest if using. Spoon into sterilized jars and cool. Refrigerate for short-term use, or process in a boiling water bath for shelf-stable storage.
Notes
For seedless jam, push warm jam through a fine mesh sieve.
Adjust sugar to taste; less sugar yields a looser jam.
To make shelf-stable jars, process in a hot water bath for 10 minutes (adjust for altitude).
A small pat of butter reduces foaming during cooking.