Cook your farro according to package directions, using either water - or for a little added flavor, vegetable broth. Drain and set aside to cool.
Meanwhile, you can cut up your apple and make the dressing. In a small bowl, mix the honey and Dijon until well-combined. Add the vinegar and olive oil and whisk until slightly thickened. Taste and season with salt and pepper.
Combine the farro, arugula, diced apple, and feta in a serving bowl. Drizzle with dressing* and toss to combine.
*You may want to start with a little dressing at a time, until you have it to your liking. We like a lot of dressing, so I use it all, but some prefer less.