Adding grated apple, a pectin-packed fruit, naturally jells the jam and gives it a pleasant tartness.
1 pound fresh (or frozen, thawed) strawberries, hulled and quartered
2/3 cup sugar
1 large Granny Smith apple, peeled and coarsely grated
1 tablespoon fresh lemon juice
Combine strawberries and sugar in a large heavy skillet. Stir in grated Granny Smith apple. Cook over medium-low heat, stirring and breaking up strawberries, until sugar dissolves. Simmer until jam is thickened, 10–15 minutes. Stir in lemon juice.
Transfer to a bowl or jar; let cool. Cover and chill until set, about 2 hours. Keep chilled; serve within 2 weeks.