Rinse mushrooms; trim and discard discolored stem ends, and thinly slice mushrooms. In a 10- to 12-inch frying pan over high heat, frequently stir mushrooms, butter, olive oil, shallot, and garlic until mushrooms are lightly browned, 6 to 8 minutes. Add port and condensed beef broth; stir often until liquid is evaporated, about 4 minutes. Add salt to taste. Use hot. If making up to 1 day ahead, cover and chill; reheat to serve. Makes 4 servings.