In an extra-large bowl, place the romaine and arugula. In a small bowl or measuring cup, make the dressing: Whisk together the olive oil, vinegar, mustard, garlic, salt, and pepper until smoothly combined (or put all of the dressing ingredients in a mason jar, seal, and shake to combine). Pour half of the dressing over the salad and toss to coat. The leaves should be lightly moistened.
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