In a small bowl, whisk the vinegar and mustard. Season with salt and pepper. Gradually whisk in the oil.
ASSEMBLE THE SALAD:
Place the lettuce and spinach in a medium salad bowl and toss together. Core the pears and apples and cut them into ½-inch dice. Place in a separate bowl and toss with the lemon juice. Add the pears and apples to the salad bowl along with the grapes, Gruyere and dressing; toss.
Sprinkle the toasted pecans on top, and serve.
Notes
HOW TO TOAST PECANS: Preheat the oven to 350°F. Spread the pecans on a baking sheet and toast in the middle of the oven for 5 to 8 minutes, or until fragrant. Keep a close eye on them so they don't burn. Let cool to room temperature, then chop coarsely.
*If preparing this recipe as gluten free, just be sure to use a brand of whole grain mustard that is designated as GF.