Heat oil in a large saucepan over medium heat. Add onion and jalapeño and cook until softened, but not browned. Add garlic and cook a minute longer. Stir in flour and cook 2 minutes more, then gradually whisk in chicken broth. Cook, stirring occasionally, until sauce simmers and thickens. Remove from heat and slowly whisk in the sour cream (do not boil after sour cream is added). Season to with salt (I use 2 tsp but start with less, in case your broth is saltier).
Cook Chicken
Heat a large pan on medium heat. Season chicken with 1 tsp kosher salt and the pepper. Add 2 tbsp oil to pan, and once it's shimmering hot, cook chicken until no longer pink inside (or registers 160º F - it’ll rise another 5º as it rests + cooks in the enchiladas), 8-12 minutes, flipping once. Chop chicken into bite-sized pieces.
Assemble
Spread 1 cup of the white enchilada sauce in bottom of a 9x13” baking pan.
Lay the tortillas on a flat work surface and divide the chicken equally among them, positioning the filling in a line along the centre. Set aside 1 cup of cheese for topping, then divide the remaining cheese among tortillas. Drizzle 12 tbsp of the sauce on top of the filling (2 tbsp per tortilla), then fold in the edges and roll them up. Nestle them into the baking pan, seam side down, as you roll them. Pour the rest of the sauce over top of the enchiladas, sprinkle with reserved cheese, cover with foil, and then bake for 25 minutes, then uncover and bake 15 minutes or until cheese is bubbling and golden.
Notes
You can add up to 2 cups of roasted/pan-cooked veggies to the filling if you like. I often make them with roasted mixed zucchini, bell peppers, and/or portobello mushrooms.