In a saucepan cover the eggs with cold water, bring the water just to a boil, and simmer the eggs for 5 minutes. Pour out the water, add cold water to the pan, and let the eggs cool until they can be handled. Shell eggs carefully and halve them lengthwise. In a small bowl stir together garlic, mustard, vinegar, and cream. Add oils in a slow stream, whisking until emulsified, and season with pepper and salt. Cut pepper into 1/4-inch dice. Prepare grill. Halve each quail lengthwise through breast and pat dry. Rub quail lightly all over with oil and season with pepper and salt. Grill quail in a hot well-seasoned ridged grill pan over moderately high heat , or until skin is well browned but meat is still pink. In a large bowl toss watercress and mushrooms with just enough vinaigrette to coat lightly. Arrange lettuce on each of 4 large plates and top with watercress mixture and bell pepper and quail eggs. Arrange quail on top of salad and sprinkle with tarragon. Serve remaining vinaigrette on the side.