Prepare a bechamel sauce: Melt butter on medium heat in a small pot. Whisk in flour until fully mixed. Slowly add milk, whisking constantly to dissolve flour/butter mixture. Add bouillon cube and stir until dissolved and mixture thickens, but do not boil.
In a separate pan, sautee zucchini, chile, and onion in oil over medium-low heat until translucent. Add garlic, cumin, and salt. Stir until fragrant (~30 sec). Add onion mixture and cooked chicken (if using) to bechamel sauce. If the chicken is not pre-salted, add an extra 1/2 tsp of salt or to taste.
In a greased 9x9" pan, spread a thin layer of sauce.
Preheat oven to 350F.
Heat tortillas on a warm comal/griddle until pliable (about 30 seconds each side). Dip tortillas in the bechamel mixture. Add to pan in layers (like a lasagna) of tortillas, sauce, cheese.
Bake at 375F for 20 minutes. Then put under broiler just until cheese is golden brown (~2-5 minutes).