Tear lettuce into small pieces. Use to line a serving bowl. Wipe mushrooms with paper towel. Trim and cut large ones in half. Finely chop rosemary and thyme leaves.
Heat oil in a large frying pan. Sauté shallots until transparent. Add mushrooms and cook until golden brown, stirring occasionally. Add chopped herbs, saving a little for garnish; sauté briefly.
Add vinegar and port wine to mushroom mixture. Cook, uncovered, until mixture is slightly reduced, about 2 minutes. Add salt and pepper to taste. Spoon over prepared salad. Spoon a little crème fraîche or light sour cream onto each portion. Sprinkle with dried cranberries.