French Country Salad

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.