Step 1

Whisk vinegar, lemon juice, agave, and mustard in a medium bowl. Whisking constantly, slowly stream in oil; whisk until emulsified. Season with salt and pepper.

Step 2

Toast walnuts in a dry small skillet over medium heat, tossing often, until slightly darkened, 5–8 minutes. Let cool.

Step 3

Toss frisée, radicchio, apple, and celery in a large bowl to combine. Season with salt and pepper; squeeze lemon juice over. Toss with enough vinaigrette to lightly coat. Transfer to a platter and top with walnuts.