In a small bowl add olive oil, sherry vinegar, chopped tarragon, salt and cracked pepper and whisk. Reserve for salad.
Brush zucchini and squash with tarragon vinaigrette and grill lightly on both sides. Reserve for salad.
In a large bowl, combine the arugula, tomatoes, and tarragon vinaigrette; add a little at a time. Do not overdress salad; greens should be lightly coated. Lay out 8 plates, arrange 1 zucchini slice and 1 yellow squash slice on each plate, add a handful of arugula tomato salad and season with salt and cracked pepper.