Prepare grill or preheat broiler. In a small bowl, stir together mayonnaise, sour cream or yogurt, and lemon juice. Brush olive oil over the mushrooms. Grill or broil mushroom slices until tender and golden, 2 to 3 minutes a side . Season with salt and pepper. meanwhile, toast bread on the grill on under the broiler. Rub both sides of the bread with garlic clove. Spread half of the mayonnaise mixture over 4 toasted bread slices and arrange basin on top. Top with the grilled mushroom slices, followed by the tomato slices and salt and pepper. Finish with a dollop of the remaining mayonnaise mixture and cover with the remaining pieces of toast.