Reduce oven temperature to 350°F. In 10-inch skillet, cook ground beef over medium-high heat, stirring frequently, until beef is thoroughly cooked; drain. Stir in taco seasoning mix and salsa; heat to boiling. Reduce heat; simmer uncovered 5 minutes, stirring occasionally. Stir in mushrooms and half of the french fried onions. Spoon mixture into baked shell.