In small bowl, mix mustard, vinegar and sugar with whisk.
Gradually add oil, beating constantly until well blended.
Season to taste with salt and pepper.
2 In large bowl, gently toss greens, tomatoes, cucumber and onion.
Pour vinaigrette over salad; toss gently to mix.
Serve immediately.
Notes / Tips / Wine Advice:
Make-Ahead Magic
You can make the dressing up to 24 hours in advance; cover and refrigerate. Stir before using. You also can slice the cucumber and onion and refrigerate in resealable food-storage plastic bags, making it even quicker to get the salad on the table when you’re ready!
Kitchen Secrets
Do you stay away from red onions because the flavor is too strong? After cutting the onions, place them in a small bowl of cold water to help mellow the flavor. Let stand at room temperature 30 minutes, or cover and refrigerate up to 24 hours. Drain and pat dry before using.