Preheat oven to 350 degrees F.
Put the chicken in a large pot and add a little more than enough water to cover the chicken. Add in the salt, pepper and sage and stir. Add in the onion, garlic and celery. Bring to a hard boil, reduce the heat to medium and boil until chicken is tender and has no red inside.
Remove the pan from the heat temporarily. Remove the chicken from the broth to a plate or bowl. Slice the chicken breasts lengthwise into about 3 or 4 strips. Cut the strips into pieces about 1/2 inch long. Put the chopped breasts back into the broth and put the broth back on medium heat. Remove the skin from the thighs and discard. Take all the meat off the thighs, discarding the bones. Be sure to trim out and discard any sinews or tendons at the ends of the thigh muscles. Cut the thigh meat into pieces about the same size as the breast pieces. Add the thigh meat back into the broth. Add the tomatoes and chopped long green chilies in and continue simmering the chicken until done. You may need to add a bit more water into the broth to have enough for the sauce - don't let it boil dry. You will need about a quart or so of broth when you make the sauce. In a small bowl, stir the milk into the flour until smooth and no lumps remain. When the chicken is done stir the milk-flour mixture into the broth slowly while stirring to prevent lumps. Remove from heat as soon as the sauce thickens a bit (don't let it get too thick).
Stack and cut about 10 tortillas at a time into halves (fewer at a time if it requires too much knife pressure). Cut each set of halves into 3 triangular pieces (each tortilla will become 6 triangles). Set aside.
Add a few of the tortilla pieces at a time into the broth, stirring well to coat the tortillas. After about a fourth to a half of the tortillas are added, stir in about half of the grated Muenster cheese. Stir in the remaining tortilla pieces, stirring to coat and moisten well. Don't add so many tortilla pieces that the sauce is too dry - In the end the sauce should still be a bit liquid.
Pour the entire mixture evenly into a large Pyrex baking dish and smooth the top. Cover evenly with the remaining half of the grated Muenster Cheese. Sprinkle the grated Yellow cheese over the top of the Muenster cheese.
Place the enchiladas into the oven for about 10 to 15 minutes or until the cheese is melted as desired.
Serve with sour cream if desired. (Saltine crackers go GREAT with this dish).
