Heat oven to 450 degrees. In large roasting pan, toss eggplant, zucchini, yellow squash and red pepper slices with oil and garlic. Season with salt and pepper. Roast vegetables, stirring occasionally, until tender and starting to brown, 30 to 35 minutes. Let cool.
Meanwhile, in small bowl, whisk together sour cream or yogurt, mayonnaise, basil and lemon juice. Season with salt and pepper; reserve in refrigerator.
Spread mayonnaise mixture on both bread halves and arrange watercress on bottom layer. Top with vegetable mixture and bread tops.