This makes a rich sauce for our simple Grilled Asparagus.
Melt butter in a medium skillet over medium heat. Add shallots and sugar; season with salt and pepper. Cook, stirring, until shallots are caramelized and tender but still retain some crunch, about 10 minutes; set aside.
In a small bowl, combine mustard and vinegar; season with salt and pepper. Slowly whisk in oil until combined; stir in shallots.