Simple Green Salad with Champagne Vinaigrette

1 cup cherry tomatoes, halved or quartered if large

1/4 cup thinly sliced red onion 

1 small head green leaf lettuce, torn or cut into bite-size pieces 

1 small head red leaf lettuce, torn or cut into bite-size pieces 

1 orange, yellow or red bell pepper, thinly sliced 

1/2 seedless cucumber, quartered lengthwise and sliced 

1/4 cup Champagne vinegar 

2 tablespoons Dijon mustard 

1/2 cup extra-virgin olive oil 

Kosher salt and freshly ground black pepper