Heat oven to 350° F. Spread the nuts on a rimmed baking sheet and toast, tossing occasionally, until fragrant, 4 to 6 minutes. Let cool, then coarsely chop.
Meanwhile, whisk together the oil, lemon juice, and ½ teaspoon each salt and pepper in a large bowl. Add the mizuna, beans, grapes, and nuts and toss to combine. Serve immediately.