Wash cauliflower and remove leaves and woody stem. Break into flowerets. (You should have about 4 cups.) Trim asparagus.
Place a steamer basket in a saucepan. Add water to just below the bottom of the steamer basket. Bring to boiling. Add asparagus or green beans. Steam asparagus, covered, for 5 to 7 minutes or just until crisp-tender (steam green beans for 18 to 22 minutes). Remove vegetables from steamer basket; chill until serving time.
Add water to just below the bottom of the steamer basket, if necessary. Return to boiling. Add cauliflower. Steam, covered, for 8 to 12 minutes or until crisp-tender. Remove cauliflower from steamer basket; chill until serving time.
For dressing, in a blender container or food processor bowl combine vinegar, mustard, and sugar. Cover and blend for 5 seconds. With machine running, add oil in a thin, steady stream through the opening in the lid. (When necessary, stop blender or food processor and use a rubber scraper to scrape sides.) Season to taste with salt and pepper. Cover and chill until serving time.
To serve, arrange green beans on one side of a large platter. Line remaining half of the platter with lettuce. Arrange cauliflower on top of the lettuce. Add several cherry tomatoes and ripe olives. Drizzle vegetables with the dressing. Garnish the platter with lemon slices and thyme, if desired. Makes 6 servings.
Steam vegetables; cover and chill for up to 24 hours. Prepare dressing; cover and chill up to 24 hours. Serve as directed.