In a small bowl, combine salsa verde and Greek yogurt to create a creamy sauce.
In a greased 12 x 8 inch baking dish, spread 1/2 cup of the sauce to cover the bottom of the dish.
Assemble the enchiladas by filling each tortilla with 1/4 cup of shredded chicken and about 2 tablespoons of cheese. Place the enchiladas in the baking dish, side by side.
Top with remaining sauce, distributing evenly over the enchiladas, then top with remaining cheese, distributing evenly as well.
Cover baking dish with aluminum foil and bake for 15 minutes.
Remove the foil and bake an additional 10 minutes, until cheese is fully melted and lightly browned.
Garnish with cilantro and diced avocado, if desired.