First, wash and sterilize your jars and lids in hot soapy water, then place them in the oven at 160°C (320°F) for 10 minutes and let them cool on clean kitchen towels.
Clean your strawberries thoroughly under cool running water. Remove the green tops and set aside ½ kg of the nicest whole strawberries for later - this will give your jam that signature chunky texture.
In a large mixing bowl, combine your sugar, lemon juice, and the remaining 1 kg of strawberries. Using clean hands or a potato masher, gently crush these strawberries until they release their juices, but don't mash them completely - you still want some texture.
Now add your reserved whole strawberries to the bowl and mix everything together gently. Transfer this entire mixture to a heavy-bottomed saucepan.
Place your pan over medium-high heat and bring the mixture to a boil, stirring occasionally. Once boiling, lower the heat to medium-low (para hindi sumunog / so it won't burn). Let it simmer for 35-45 minutes, stirring every few minutes to prevent sticking. The jam should reach 105°C (220°F) on a thermometer.
To test if your jam is ready, place a small spoonful on a cold plate. Let it cool for a minute, then push it gently with your finger. If the surface wrinkles and the jam doesn't run, it's ready. If it's still runny, continue cooking for a few more minutes and test again.
Once ready, carefully pour the hot jam into your prepared sterilized jars, leaving about ¼ inch of space at the top. Wipe the rims clean with a damp paper towel, then seal the lids tightly.
Let the jars cool completely at room temperature. You'll hear a satisfying 'pop' sound as each lid seals. Label your jars with the date (lagyan ng petsa), then store them in a cool, dark place or in the refrigerator.
For the best flavor and texture, let your jam rest for 24 hours before opening. Once opened, keep refrigerated and use within one month. Unopened jars will keep for up to 6 months in a cool, dark place.