Preheat the oven to 350ºF (175ºC), and lightly toast the walnut pieces on a baking sheet for about 4 minutes. Cool and reserve. In a small bowl, whisk together the lemon juice and salt. Let sit for 5 minutes before whisking in the olive oil, zest, pepper, and mint.
In a larger bowl, gently but thoroughly toss the lettuce and apple batons with the vinaigrette. Plate and top each serving with walnuts and Gruyère matchsticks.
Cook's Tip
If preparing ahead, submerge the pieces of apple in water and several teaspoons of freshly squeezed lemon juice for up to 30 minutes. This prevents the apples from turning brown. Just drain and pat dry when ready to add to the salad.
Photo by Doug Bradshaw.