Make a simple raspberry puree by combining 12 ounces of fresh raspberries, 1/2 cup of sugar and 1/4 cup of water in a small saucepan. Cook the mixture over high heat until it boils. Reduce the heat to medium, and let the mixture cook until the berries fall apart and the puree is thickened. Use a blender to make the puree smooth, or run the puree through a strainer to remove the seeds.
Let the mixture cool for several minutes before pouring it into the blender. With the cap tightly sealed, run the blender for several seconds to give the puree an even texture.
Alternatively, set a fine mesh strainer over a bowl. Pour the puree into the strainer, and push down on it with a wooden spoon. Discard the seeds that are left in the strainer.
Add 1 tablespoon of lemon juice to the finished puree to enhance the tartness of the berries, or stir in 1/4 teaspoon of vanilla extract.
Adjust the amount of sugar used based on how sweet or tart the raspberries are. Two tablespoons of sugar may be sufficient if the berries are naturally very sweet, or if the puree is to be used as part of a dish that is already very sugary.