In 10-inch nonstick skillet, cook beef and onion over medium-high heat 5 to 7 minutes, stirring frequently, until beef is thoroughly cooked; drain. Stir in taco seasoning mix and water; heat to boiling. Reduce heat; simmer uncovered 2 to 3 minutes, stirring frequently, until mixture has thickened. Remove from heat.
To assemble, spread about 2 tablespoons refried beans in bottom of each shell. Top each with heaping 1/4 cup beef mixture and 2 tablespoons shredded cheese. Bake 5 to 7 minutes or until cheese is melted. Top with remaining ingredients, if desired.