Wash the jars, lids and rings with hot, soapy water, making sure to rinse well. To sterilize the jars and keep them hot, place them on a baking sheet in a preheated 175°F (79°C) oven OR in your water bath canner while cooking the jam. Place a small glass plate in the freezer.
In a large bowl, crush the strawberries and stir in the sugar. Let sit for 1 hour to allow the berries to release their juices. (Or up to 24 hours in the fridge.)
Add the lemon juice and grated apple. Transfer the mixture to a large stockpot and bring to a boil over medium-high heat. Once it reaches a boil that can't be stirred down, set a timer for 20 minutes. Turn off heat.
Test for doneness - Remove the glass plate from the freezer and drop a spoonful of the hot jam on the plate. Put the plate back into the freezer for 2 minutes. Remove the plate and run your finger through the jam. If the line stays separated, your jam is ready to be jarred. If the line closes, boil the jam for 5 more minutes and repeat the test.
Ladle the hot strawberry jam into clean, sterilized jars, leaving a ¼ inch headspace at the top. Remove air bubbles and wipe the rim of the jar to remove any residue. Center the canning lid on the jar and adjust the band to fingertip-tight.
If not canning, let the jars cool completely, tighten bands and store in the refrigerator for up to 2 months.