Step 1

Cook lentils and quinoa in a large saucepan of simmering salted water until lentils are tender but still retain their shape, 15–20 minutes. Drain; transfer to a medium bowl.

Step 2

Whisk vinegar, lemon juice, and mustard in a large bowl. Whisking constantly, gradually stream in oil; whisk until emulsified. Season with salt and a generous amount of pepper. Add lentils and quinoa, celery stalks, cabbage, celery leaves, and mint and toss to coat. Add cheese and toss again. Taste and season with more salt and pepper if needed.

Step 3

Do Ahead: Lentils and quinoa can be cooked 1 day ahead. Let cool; cover and chill.