Ingredients
1 tsp. Dijon mustard
1 egg yolk
1 cup canola oil, plus more for frying
2 tsp. white wine vinegar
2 tsp. fresh lemon juice
1⁄2 tsp. kosher salt, plus more to taste
Freshly ground black pepper, to taste
2 lb. Yukon Gold potatoes, peeled and cut into 1⁄4"-thick sticks
2 1⁄2 lb. mussels, debearded and scrubbed
2⁄3 cup dry white wine
2 tbsp. unsalted butter, cubed
3 ribs celery, finely chopped
1 1⁄2 leeks, light green and white parts, cut into 1⁄4"-thick slices
1⁄2 large yellow onion, finely chopped