4 cups blueberries
1/2 cup water
3 1/2 cups sugar
1 tablespoon lemon juice
Preheat oven to 250 degrees.
Put a plate in the freezer.
With a potato masher or spoon, crush 1 cup berries in a medium saucepan. Add remaining 3 cups berries and 1/2 cup water; bring to a simmer over medium heat. Cook, stirring occasionally with a wooden spoon to break up berries, about 6 minutes.
Meanwhile, spread sugar out on a rimmed baking sheet. Heat in oven until warm, about 5 minutes.
Slowly stir sugar into saucepan; return mixture to a simmer, stirring constantly. Reduce heat to medium-low; cook at a slow boil until thickened, 45 minutes to 1 hour.
Once preserves reach 228°F, start checking if it is properly set. Conduct the plate test: Put a spoonful of preserves on the chilled plate, and place in the freezer until preserves reach room temperature, about 1 minute. Draw your finger through preserves. If it immediately runs back together, it is not ready. Once preserves are ready, remove from heat and stir in lemon juice.
Makes about 4 cups.