Prepare Mustard-Herb Dressing.
In a saucepan, cook asparagus, covered, in a small amount of boiling salted water for about 4 minutes or until crisp-tender. (Or steam asparagus for about 4 minutes.) Immediately drain and plunge into ice water. Drain.
In a saucepan, cook squash, covered, in a small amount of boiling salted water for about 3 minutes or until crisp-tender. (Or steam squash for about 3 minutes.) Immediately drain and plunge into ice water. Drain.
In a salad bowl, combine the baby salad greens or other greens, asparagus, and yellow squash. Sprinkle with sunflower seeds. Toss with Mustard-Herb Dressing to coat. Makes 8 side-dish servings.
In a blender container, combine vinegar, tarragon, mustard, sugar, and pepper. With blender running, add oil in a thin, steady stream. Continue blending until mixture is thick. Cover and store in the refrigerator for up to 1 week. Makes 2/3 cup dressing.