Whisk garlic, vinegar, mustard, and a light sprinkling of salt and pepper in a 1-cup Pyrex measuring cup. Slowly whisk in olive oil to make thick vinaigrette.
Meanwhile heat 1½ teaspoons of the vegetable oil in a large saucepan or small pot over strong medium heat. Add popcorn and cook, shaking pan frequently to ensure even popping, until corn is completely popped. Pour popped corn into a large bowl; drizzle with remaining tablespoon of oil and sprinkle with ½ teaspoon salt; toss to coat. Add arugula and onion to the bowl and drizzle with vinaigrette to taste; toss to coat again. Serve immediately.
Recipe by Three Many Cooks at https://threemanycooks.com/recipes/salads-and-sides/popcorn-salad/