Combine the cherry tomatoes, onions, green leaf lettuce, red leaf lettuce, bell peppers and cucumbers in a large salad bowl.
In small bowl, whisk together the vinegar and mustard. Gradually whisk in the oil to emulsify. Season with 1/2 teaspoon salt and a few grinds of pepper.
Toss the salad with vinaigrette and season with additional salt and pepper as needed.