Preheat the oven: Preheat your oven to 375°F (190°C).
Prepare the filling: In a large skillet, heat olive oil over medium heat. Add diced onion and minced garlic. Cook until the onion is soft and translucent, about 5 minutes. Add chopped spinach and cook until wilted—season with salt, ground cumin, onion powder, and chili powder. Stir in shredded chicken, 3/4 cup shredded cheese, diced green chiles, and Greek yogurt. Mix well and remove from heat.
Make the sauce: In a saucepan, melt butter over medium heat. Stir in all-purpose flour and cook for 1-2 minutes until lightly golden. Gradually whisk in chicken broth until smooth. Bring to a simmer and cook for 5 minutes, stirring occasionally, until the sauce thickens. Remove from heat and whisk in Greek yogurt until smooth—season with salt and pepper to taste.
Assemble the enchiladas: Spread a small amount of the Greek yogurt salsa verde sauce in the bottom of a 9x13 inch baking dish. Spoon the chicken and spinach filling into each flour tortilla, roll up tightly, and place seam side down in the baking dish.
Pour the sauce: Pour the remaining sauce over the top of the enchiladas, spreading evenly to cover them.
Top with cheese: Sprinkle the shredded Monterey Jack cheese over the top of the enchiladas.
Bake: Cover the baking dish with foil and bake in the preheated oven for 20-25 minutes, or until the enchiladas are heated through and the cheese is melted and bubbly.
Serve: Remove from the oven and let the enchiladas cool for a few minutes before serving. Garnish with avocado slices, feta cheese and fresh cilantro if desired.