Combine diced apples, sugar, and lemon juice in a large bowl. Mix gently and refrigerate for at least 4 hours or overnight.
Before cooking, place a small plate in the freezer for gel testing later.
Strain the macerated fruit, reserving the liquid. Boil the reserved liquid in a saucepan, then add the apple pieces.
Cook over medium-high heat for 20-30 minutes, stirring occasionally to prevent scorching.
To test for gel consistency, remove the frozen plate from the freezer and place a small amount of jam on it. Return the plate to the freezer for 1 minute. If the jam wrinkles when pushed with your finger, it's ready. If not, continue cooking and test again in 5-minute intervals.
Once desired consistency is reached, remove from heat.
Fill sterilized jars with hot jam, leaving 1/4-inch headspace.
Process in a water bath canner, by boiling the jars in hot water for 10 minutes, then let sit in the hot water for an additional 5 minutes.
Remove jars and let cool completely before checking seals.