Green Chili Sauce
Melt the butter in a large saucepan over medium heat. Add the onion and sauté until soft, 6 to 7 minutes. Add the garlic and sauté 2 minutes longer. Whisking continuously, sprinkle in the flour and continue whisking for about 2 minutes. Still whisking, slowly pour in the stock, continuing to whisk until the sauce begins to thicken. Add the chilies, salt, and cumin. Reduce the heat and simmer for 10 minutes. Add the 1 cup diced chicken and simmer, stirring occasionally, for 10 minutes longer. Cover and keep warm.

Enchiladas
Preheat the oven to 350°F. Put the tortillas on a microwave-proof plate, cover with a damp kitchen towel, and heat in the microwave for 30 seconds to soften.

Arrange the tortillas on a work surface and distribute down the center of each the diced chicken, shredded cheeses (reserving ½ cup of each), red peppers, and diced jalapeños. Roll up each tortilla and carefully place seam down side by side in a greased 9-by-12-inch baking pan. Sprinkle ¾ cup of the reserved cheese over the top.  Bake for 20 minutes. Then, spoon three quarters of the Green Chili Sauce over the tortillas and top with the remaining cheese. Continue baking until the cheese has melted, about 10 minutes longer. Use a spatula to transfer to heated serving plates, garnishing each portion with sour cream, a spoonful of the remaining sauce, and a border of chopped lettuce.