Mussels and Fries with Mustard Mayonnaise

Cook the fries according to package instructions and keep warm in the oven if necessary.

Meanwhile, cut the onion into very thin slices with the slicer, then cook with garlic and a pinch of salt in the butter in a wide heavy medium pot over medium-high heat, covered, stirring occasionally, until pale golden.

While the onion cooks, whisk together the mustard and water until smooth, then whisk in the mayonnaise and about 1/4 teaspoon pepper.