In a medium skillet, SAUTE onion in olive oil over medium high heat until tender, about 5 minutes.
In a mixing bowl, STIR together chicken, 1/4 cup sour cream and 1 1/2 cups shredded Monterey jack; STIR in onion.
SPOON chicken mixture evenly on one end of each tortilla; ROLL up.
In a lightly greased 13 x 9-in baking dish, ARRANGE roll-ups, seam side down.
In a saucepan over medium heat, WHISK together the 2 cans of green chiles, milk and flour.
Gradually ADD remaining 2 1/2 cups of shredded Monterey jack, STIRRING constantly, until cheese is melted and mixture is smooth.
POUR sauce evenly over top of enchilada; SPREAD remaining 1 3/4 cups of sour cream over sauce.
BAKE enchiladas at 400° until bubbly, about 20 to 25 minutes.
REMOVE from oven; SPREAD remaining can of green chiles evenly over top of baked enchiladas.
Per Serving (excluding unknown items): 1593 Calories; 111g Fat (62.3% calories from fat); 98g Protein; 53g Carbohydrate; 2g Dietary Fiber; 396mg Cholesterol; 2451mg Sodium. Exchanges: 2 1/2 Grain(Starch); 22 Lean Meat; 1 Vegetable; 1/2 Non-Fat Milk; 28 1/2 Fat.