Wash and hull the strawberries. Slice them into roughly 1/2-inch pieces. If using, zest the lemon before juicing.

In a large, heavy-bottomed saucepan, combine the sliced strawberries, sugar, lemon juice, lemon zest (if using), and butter (if using). Gently stir to combine.

Bring the mixture to a boil over medium-high heat, stirring frequently to prevent sticking.

Once boiling, reduce the heat to medium-low and continue to stir occasionally. Cook for 10-15 minutes, or until it reaches setting point (see note below).

To check for setting point: Place a small amount of jam on a chilled plate. Let it cool slightly, then push it with a finger. If it wrinkles, it's ready. Alternatively, use a jam thermometer; the jam should reach 220°F (104°C).

Ladle the hot jam into sterilized jars, leaving about 1/4 inch of headspace. Wipe the rims clean, seal with lids and rings.

For longer shelf life, process the jars in a boiling water bath for 10 minutes. Cool completely. Check seals; you should hear a 'pop'. Store sealed jars in a cool, dark place. Refrigerate unsealed jars.