Peel the apples and remove the core.
Cut into small pieces, add sugar, stir, leave for 15 minutes.
Place the apples over low heat and simmer for 20 minutes. Stir constantly, without moving away from them, otherwise they will burn.
Soften the washed lingonberries slightly with an immersion blender or masher. You don't need to crush them completely, just enough to release the juice; the berries will eventually cook to the desired consistency.
Add the lingonberries to the apples, stir well, and cook for another 30 minutes, stirring constantly. Don't forget to skim off any foam. To check if the jam is ready, drop it onto a saucer—it doesn't run, meaning it's ready.
Place the jam into jars while it is hot; it will thicken after cooling.
For winter storage, sterilize the jars first. If you don't plan to store it for long, simply refrigerate it; it will keep there for 2-3 months.