1 cup loosely packed basil leaves, torn into shreds (if large)
2 tomatoes, sliced
Preheat grill or broiler.
Stir mayonnaise, sour cream (or yogurt) and lemon juice in a small bowl.
Brush oil over cut sides of mushrooms. Grill or broil the mushroom slices until tender and golden, 2 to 3 minutes per side. Season with salt and pepper. Toast bread on the grill or under the broiler. Rub both sides of the bread with the cut sides of the garlic clove.
Spread half of the mayonnaise mixture over 4 toasted bread slices and arrange basil on top. Top with the grilled mushroom slices and tomato slices; season with pepper. Finish with a dollop of the remaining mayonnaise mixture and cover with the remaining pieces of toast. Cut sandwiches in half and serve immediately.