To make the vinaigrette, combine the vinegar, dry mustard, and shallot in a small bowl. While whisking, slowly add in the olive oil and whisk until emulsified. Add a pinch of sea salt and some black pepper. Whisk. Taste and season with more salt and pepper if necessary.
To assemble the salad, place stone fruit and baby arugula in a large bowl. Slowly drizzle a little dressing over the salad and start to gently mix. Add dressing until just the leaves of the arugula are lightly coated. With a vegetable peeler, start shaving long pieces of pecorino into the salad. Mix and continue shaving until you have a good amount in the salad. Remember pecorino is salty so taste as you go. Give one final gentle toss and serve immediately.