Melt butter in a medium saucepan over low heat. Still cooking, add milk, salt and chicken broth. Gradually add flour, stirring until blended. Increase heat to medium. Add chiles. Cook, stirring, until sauce has a creamy texture (about 10 minutes). Makes 3 cups. Set aside to use for topping enchiladas.
Preheat oven to 350ºF.
Spread 1/2 cup of warm chile cream sauce in the bottom of a 2-quart baking dish and set aside rest of sauce, keeping it warm.
In a bowl, mix shredded zucchini and 1 1/2 cups of the cheese. Set aside.
Warm tortillas so they are pliable (do this according to package directions, or use whatever method you prefer). One by one, dip warmed tortillas in the pan of remaining sauce and remove immediately (if allowed to sit in the sauce, the tortilla will start to dissolve). Move each dipped tortilla to the baking dish and put 1/6 of the reserved filling mixture in the center of the tortilla. Roll each tortilla and arrange in the baking dish, seam side down. Repeat until all the tortillas and filling are used up. Cover enchiladas with remaining sauce and 1/2 cup cheese. Bake for approximately 20 minutes at 350ºF, or until cheese is melted and sauce is bubbly. Remove from oven and top with chopped onion, crema/sour cream and cilantro, if using.