Preheat oven to 350 degrees.
In a large skillet, melt the butter over medium heat. Add the onions and cook, stirring, until soft, about 3 minutes. Add the pork, beef, ancho chile ancho powder, oregano, cayenne, and thyme. Sauté over medium heat for 5-8 minutes until meat is browned and cooked through.
Lightly grease a large baking sheet. Arrange halved poblano peppers in a single layer on the baking sheet and bake for 15 minutes.
Add tomato sauce, chipotle pepper, and adobo sauce to meat mixture and mix to combine well. Simmer for 10 minutes to reduce sauce and allow flavors to meld.
Spoon mixture into the peppers, sprinkle with cheese, and return to oven for another 10 minutes or so until peppers are tender and cheese is melted. Allow to cool slightly before serving.