n a small skillet over medium heat, toast peach slices and walnuts (no oil) for about 2 minutes, turning occasionally until the peaches are warm and lightly golden and the walnuts are fragrant.
Set aside to cool slightly.
In a small bowl or jar, whisk together Dijon mustard, vinegar, lemon juice, minced red onion, salt, and pepper.
Slowly drizzle in olive oil while whisking until the dressing becomes smooth and emulsified.
Taste and adjust — add more mustard for bite or vinegar for brightness.
Arrange mixed greens in a bowl or on a plate. Add toasted peaches, walnuts, and red onion.
Crumble goat cheese over the top and drizzle with vinaigrette (about 2 tablespoons).
Toss gently and serve immediately.