To make the Dijon vinaigrette, add olive oil, mustard, honey, lemon juice, salt, and pepper to a small jar. Put the lid of the jar on and shake until thoroughly combined.
Break apart lettuce and add to a large bowl along with radishes and red onion. Pour desired amount of dressing over lettuce and vegetables and toss gently to combine. Store any leftover dressing in the fridge for up to a week.