In a large non-stick skillet, heat oil over medium-high heat. Cook chicken, covered, until browned and cooked through, about 12 min. Cool slightly and shred. Transfer to a bowl and mix with onion and salsa.
Divide mixture evenly between tortillas. Sprinkle with cheese. Fold in sides and roll up to enclose filling. Wrap individually in parchment paper and store in a resealable plastic freezer bag to freeze.
To serve, microwave one at a time on HIGH until the filling is hot, about 2 min. from frozen or 1 min. from fresh. Let stand at least 2 min. before eating. Alternatively, remove parchment, wrap in foil and bake in a 350°F (180°C) oven for about 20 min. from frozen, or cook unwrapped in a panini press, grill pan or skillet until tortilla is golden, filling is hot and cheese is melted, about 8 min. from fresh.
To reduce prep time, use a pre-cooked rotisserie chicken or leftovers from the refrigerator. To wrap, place a strip of filling down centre of tortilla. Fold up bottom and hold. Fold one long side over filling and hold. Fold over other long side making sure folds are tight. NOTE: Total Time: 27 to 43 min. from frozen, 26 to 31 min. from fresh.