Preheat oven to 400 degrees F and line a large baking sheet with parchment paper. In a large mixing bowl toss cubed butternut squash with olive oil and sprinkle with salt and pepper. Spread the butternut squash in an even layer onto the prepared baking sheet and roast for approximately 20 minutes, or until butternut squash is fork tender (but NOT mushy). Remove from the oven and set aside to cool.
Meanwhile, as the butternut squash is roasting add the dressing ingredients to a small jar or bowl and whisk until creamy and fully combined. Set aside.
In a large salad bowl gently toss together the cooked lentils, beets, grapes, and shallots. Add the arugula, spinach, mint, roasted butternut squash and dressing and gently toss one more time.