In a small saucepan, combine the brown sugar, cornstarch, and salt, stirring them together so the dry ingredients are evenly distributed.
Add the undrained crushed pineapple, lemon juice, and Dijon mustard to the saucepan. Stir thoroughly until the mixture is fully blended and uniform in texture.
Set the saucepan over medium heat and cook, stirring continuously as the contents warm. As the sugar dissolves and the sauce heats through, the mixture will begin to thicken.
Bring the sauce to a boil while continuing to stir. Once boiling, cook for 1 additional minute so the sauce thickens properly and the cornstarch is fully cooked.
Remove the saucepan from the heat and serve the sauce warm alongside ham.