1/4 teaspoon freshly ground black pepper
1/4 cup whole grain Dijon mustard
2 tablespoons apple cider vinegar
2 tablespoons fresh lemon juice
1 tablespoon sugar
1/4 cup vegetable oil
1 1/2 pounds brussels sprouts, trimmed
Whisk mustard, vinegar, lemon juice, and sugar in small bowl; whisk in oil. Season with coarse salt and pepper.
Bring large pot of water to boil. Add 1 tablespoon coarse salt. Add brussels sprouts. Cook until crisp-tender and still bright green, about 5 minutes. Drain; rinse with cold water. Cool on paper towels. Using processor fitted with 1/8- to 1/4-inch slicing disk, slice brussels sprouts. Transfer to large bowl.
Toss brussels sprouts with enough dressing to coat. Let marinate 30 to 60 minutes.