Preheat oven to 400 degrees.
Combine tomatillos, garlic cloves, and jalapenos on a baking tray. Roast for 12-15 minutes. Toss into a food processor and process until ingredients are combines and sauce is chunky.
Reduce oven heat to 350 degrees.
Heat olive oil in a large skillet over medium high heat. Add onion and cook for 5-7 minutes, or until onion is translucent.
Add garlic and cumin and cook for one minute more. Gradually add flour and then add chicken stock. Stir to ensure that flour is well combined and mixture does not get lumpy. Continue to cook for a few minutes more until liquid begins to thicken.
Remove pan from the heat. Stir in roast chicken and half of the salsa verde sauce. Season with salt and pepper.
Spread about 1/4 - 1/2 cup salsa verde sauce on the bottom of a 13 x 9 x 2-inch baking dish.
Wrap tortillas in plastic wrap and microwave them for 10-15 seconds until tortillas are soft and pliable. Lightly coast each tortilla with a small amount of salsa verde sauce.
Arrange the chicken mixture down the center of each tortilla. Sprinkle with shredded cheese.
Roll each tortilla and place seam side down in the baking dish. Pour remaining salsa verde over enchiladas and sprinkle with remaining cheese.
Bake uncovered for 30 minutes until cheese is melted.