For this French-style composed salad, sliced ham and sauteed apple are presented with Gruyere, balsamic onions, rustic bread, and Boston lettuce dressed in tangy Dijon vinaigrette.
Make the dressing: Whisk champagne vinegar, mustard, and 1/2 teaspoon salt in a medium bowl. Gradually add oil, whisking until emulsified. Stir in walnuts. Season with pepper. Set aside.
Stir onions, balsamic vinegar, and 1/2 teaspoon salt in a skillet; season with pepper. Bring to a boil. Reduce to a simmer. Cook, stirring occasionally, until onions have absorbed most of the liquid, 8 to 10 minutes. Drain. Let onions cool.
Melt butter in a medium skillet over medium-high heat. Add apple; cook, flipping once, until just beginning to brown and soften, 1 to 3 minutes per side. Remove from heat; set aside.
To serve, arrange 2 slices of ham on each of 4 plates. Toss lettuce with dressing; divide among plates. Arrange cheese and apple slices on top, evenly dividing among plates. Garnish each plate with 2 onions. Serve with bread.