Combine garlic clove, lemon juice, vinegar, mustard and 1/2 teaspoon of kosher salt in a jar or large glass measuring cup. Whisk in olive oil, then season with black pepper and more salt to taste. Set aside to let garlic marinate while you assemble the salad.
In a large bowl, combine snap peas, radishes, arugula and feta.
Remove garlic clove from dressing and discard. Add two-thirds of dressing to salad then toss to combine. Taste and add more dressing if desired. Serve.