Preheat oven to 350°F (175°C).

Hull strawberries and cut in half.

Place strawberries in a large bowl and mash with a potato masher until you have 3 cups of crushed berries and juice.

Stir in the sugar.

Pour the mixture into a 13x9-inch (3-quart) baking dish.

Place rosemary sprigs on top and press into the strawberry mixture.

Bake for 50 minutes, or until the sugar is totally dissolved and the mixture is thickening, stirring once or twice when the mixture starts to bubble.

To test for thickness, place a small ceramic plate in the freezer for 10 minutes.

Spoon a small amount of jam onto the cold plate and tilt it.

If the jam sets up as it flows down the plate, it is thick enough.

If you prefer a thicker jam, return to the oven and roast for 5 minutes longer.

Remove from the oven and discard the rosemary sprigs.

Ladle the hot jam into 4 clean (1/2-pint) freezer-safe jars.

Wipe the rims of the jars and seal.

Cool at room temperature for 24 hours.

Store in the refrigerator for up to 3 weeks or in the freezer for up to a year.