Roast the pumpkin according the directions (linked above). Scoop the roasted pumpkin out of the skin and put it all in a 1-quart saucepan. Stir in the brown sugar, cinnamon, ginger, allspice, and nutmeg and cook over medium heat until the sugar dissolves and the mixture is heated through. Blend with the immersion blender or in a jar blender until smooth. Store in a jar in the refrigerator for up to 3 weeks or in the freezer for up to 3 months.