Fill a large pot with water. Place a glass jar on a rack in the pot or place a plate in the bottom of the pot---face up--- and submerge the glass jar, the lid and screw top. Bring to a boil and sterilize the jar for about 10 minutes. (The rack and plate prevents the jar from shattering if the bottom of the pot gets too hot.)
While the water is being heated, wash gooseberries and pinch off stems. Mix berries, sugar and juice of half a lemon in a saucepan. (If you prefer less sugar, adjust the amount.) As the berries cook, mash them. Cook for about 10 minutes. Take off heat and infuse the jam with mint leaves for a few minutes. Then discard the mint.
Fill the sterilized jar with the jam. Screw the top, but not super tight. Place back in the water bath and bring to a boil. Sterilize for 10 minutes. Remove the jar and let cool before placing it in the refrigerator.
The jar is sealed if the lid is lowered in the middle. The lid will pop when you open the jar. Enjoy the jam right away, or save for later.