Combine 1 pound fresh (or frozen, thawed) hulled, quartered strawberries and 2/3 cup sugar in a large heavy skillet. Stir in 1 peeled, coarsely grated large 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 1 tablespoon fresh lemon juice. Transfer to a bowl or jar; let cool. Cover and chill until set, about 2 hours. Keep chilled; serve within 2 weeks.