Heat oven to 350 degrees. Spread walnuts on baking pan. Bake until fragrant, about 10 minutes. Set walnuts aside to cool.
Place Concord grapes in the bowl of a food processor, and puree. Pass puree through a fine sieve set over a small bowl, and discard solids. Add walnut oil and lemon juice to the bowl, and whisk to combine. Season with salt and pepper; set grape dressing aside.
Fill a large bowl with cold water. Squeeze lemon halves into water; add rinds. Using a mandoline or a sharp knife, julienne apples. Place apples in acidulated water. Set aside.
When ready to serve, drain apples. Combine apples, green and red grapes, and dressing in a large bowl; toss gently to coat. Top with reserved toasted walnuts.