Preheat your oven to 375F and move the rack to the top third of the oven.
In a medium bowl, combine the chicken with half of the shredded cheese and some salt & pepper. Working one at a time, fill each tortilla with an equal amount of the mixture, roll it up tightly, and place it seam-side down in a lightly greased 9x13 baking dish (I use Pam spray).
Cut the cream cheese into cubes and microwave it until very soft.
In a large skillet, melt the butter over medium-high heat. Add the onion and sauté for 5-7 minutes or until it's softened and lightly browned.
Stir in the garlic and cook for about 30 seconds.
Add the cream cheese, half-and-half, and chicken broth, and whisk until the cream cheese has been incorporated (this will take a few minutes).
Stir in the diced green chilies and chili powder. Let it cook for a few more minutes (turn the heat down if needed so it doesn't boil like crazy). The sauce should lightly coat the back of the spoon but not be too thick. It'll thicken up a bit more in the oven.
Pour the sauce over the tortillas (I like to leave the edges of the tortillas free of sauce so they crisp up a bit), then top with the remaining cheese.
Bake, uncovered, for 20 minutes or until hot and bubbly. If needed, broil for a couple minutes to brown the cheese (watch it carefully!).
Serve with chopped cilantro and scallions and enjoy immediately! Season with extra salt & pepper if needed.