Heat over medium-high and cook cubed pork in batches. Salt and pepper each batch. Be sure not to crowd. Brown each side and transfer to a plate or bowl.
Add additional olive oil if necessary and brown onions and poblano peppers (about 5 minutes). When slightly softened, add garlic, ancho chili powder, cumin and Mexican oregano.
Cook until fragrant, about 1 to 2 minutes. Add pork back to the pan and add pureed tomatillos (*all but 1 1/2 cups) chicken stock (*all but 1 cup), and diced tomatoes.