Remove stems from mushrooms and finely chop stems; set caps aside. In a large nonstick skillet coated with cooking spray, saute onions and chopped mushrooms until crisp-tender. Add 1/2 cup broth; cook and stir until liquid is evaporated and vegetables are tender, about 10 minutes.
Meanwhile, in a small saucepan, bring remaining broth to a boil. Stir in couscous. Remove from the heat; cover and let stand for 5-10 minutes or until broth is absorbed. Fluff with a fork. Add to onion mixture. Stir in 2 tablespoons cheese, mayonnaise, basil and tarragon. Stuff into mushroom caps.
Place on a foil-lined baking sheet. Sprinkle with remaining cheese. Bake at 375° for 10-15 minutes or until mushrooms are tender. Just before serving, sprinkle with paprika.