Mix 1/4 cup mustard, the oil, dill, garlic, 1 1/2 teaspoons salt, and 3/4 teaspoon pepper in a medium bowl. Add chicken and onion, and toss to coat. Refrigerate, covered, for 30 minutes.
Preheat grill to medium-high. Grill onion, turning often, 8 to 10 minutes. Grill chicken on 1 side for 3 minutes; flip, top with cheddar, and cook 3 minutes more. Cut cutlets in half. Grill baguette, cut sides down, until crisp, 1 to 2 minutes.
Mix remaining 3 tablespoons mustard and the creme fraiche in a small bowl. Spread onto cut sides of baguette. Sandwich chicken, pickles, and onion between bread.