Wash the hawthorn berries thoroughly under cold water and place in a pot with enough water to cover the bottom. Bring to a gentle simmer over medium heat.

Use a potato masher to mash the berries after they have softened, aiming for a pulpy consistency.

Strain the mixture through a jelly strainer or fine cheesecloth to collect the juice, allowing it to drain for about 15-20 minutes.

In a large saucepan, combine the strained juice with sugar and lemon juice, stirring over low heat until the sugar dissolves.

Increase heat to medium-high and boil for about 10 to 15 minutes while stirring frequently until mixture thickens.

Carefully pour the jelly into sterilized jars, leaving about 1/4 inch of headspace, and wipe the rims clean.

Process the jars in a boiling water bath for 5 minutes to create a vacuum seal, then let them cool completely.

Allow the jelly to set at room temperature for a full day before storing.