1. Combine the lettuces, bell pepper, cucumber, tomatoes, and onion in a large salad bowl.
2. In small bowl, whisk the vinegar and Dijon together. Gradually whisk in the oil to emulsify and season with 1.4 teaspoon salt and a few grinds of pepper. Toss the salad with the vinaigrette and season with additional salt and pepper if desired.