. Make the green sauce
Heat oil in a saucepan over medium heat.
Whisk in flour and cook 1–2 minutes until bubbly and lightly golden.
Slowly whisk in chicken broth, stirring constantly to keep it smooth.
Simmer 3–5 minutes until slightly thickened.
Stir in the mild green enchilada sauce and heat through. Set aside.
If the sauce is too thick, add a little more broth. If too thin, simmer a few minutes longer.
Make the filling
Heat olive oil in a large skillet.
Add onion and cook 2–3 minutes until soft.
Add zucchini, yellow squash, and garlic; cook about 5 minutes until tender but not mushy.
Stir in chopped spinach and cook just until wilted.
Season with salt and pepper.
Remove from heat and mix in cilantro and 1 cup shredded Casero cheese.
Assemble
Preheat oven to 375°F.
Spread a thin layer of green sauce in a greased 9×13 dish.
Warm tortillas (20–30 seconds wrapped in a damp towel).
Spoon filling into each tortilla, roll, and place seam-side down in the dish.
Pour remaining sauce over enchiladas and top with cheese.
Bake
Bake uncovered 20–25 minutes until bubbly and lightly golden.
Garnish with fresh cilantro.