First make the avocado cream sauce: Melt the butter in a skillet over medium high heat. Add the flour, whisking until golden and bubbly, about 2-3 minutes. Slowly whisk the broth into the flour mixture. Bring to a boil, then reduce heat to medium-low and simmer for 5 minutes. Stir in the sour cream, cumin, salt, garlic powder and pepper, whisking if necessary to remove any lumps. Remove from heat and transfer mixture to a blender or food processor. Add avocados, cilantro and lime juice, and pulse until smooth and well-blended. (Be very careful blending hot liquids — the heat will expand! So be sure to remove the lid occasionally so that the heat can escape.) Season with additional salt or pepper if needed.
In a large skillet, heat olive oil over medium-high heat. Add onion, poblano and jalapeno, and saute for 5-6 minutes until the onions are cooked and translucent. Remove from heat.
To assemble the enchiladas, place a tortilla on a flat surface. Spread a tablespoon or two of the avocado sauce down the middle of the tortilla. Then layer on some of the vegetable mixture, shredded chicken (or black beans if making vegetarian) and cheese. Carefully roll the tortilla and place it seam side down in the baking dish. Repeat with the remaining tortillas. Drizzle the top with about half of the remaining avocado cream sauce. Add a sprinkling of extra cheese. Cover the dish with foil and bake for about 20 minutes, or until tortillas are heated through and begin to harden. Remove the foil and broil for a few minutes until cheese is browned and bubbly.
Remove from the oven, then serve individual enchiladas drizzled with the remainder of the avocado cream sauce. You can also garnish with additional cilantro, cheese, and/or sour cream.