In a large skillet, cook beef and onion over medium heat until no longer pink; drain. Stir in the chili powder, cumin, coriander, salt, oregano, garlic powder and paprka.
Combine cornstarch and water until smooth. Stir into beef mixture. Bring to a boil; cook and stir for 2 minutes or until thickened. Serve with taco shells. Garnish with toppings if desired.
In a small saucepan, cook refried beans over medium-low heat for 2-3 minutes or until heated through.
Spoon about 1/3 cup of beans off-center on each tortilla; top with about 2 rounded tablespoons of beef mixture. Sprinkle with cheese. Fold sides and ends of tortilla over filling and roll up. In a large skillet over medium-high heat, brown burritos in oil on all sides. Serve with sour cream and salsa.