I prepared the poblano sauce for these enchiladas in my vitamix, but you can also use a regular blender or food process. Just puree the ingredients until smooth (juice the lime instead of adding it whole) and heat the sauce in a pan until warm. I added a jalapeño because I like things spicy, but you can leave it out if you don't want the extra heat.
Heat your oven to 350ºF. Rub the chicken breasts with chili powder.
Heat the olive oil in a large, deep-sided skillet. Add the chicken and cook until just browned, 1-2 minutes per side. Add the chicken stock and let the chicken simmer for 10 minutes or until cooked through and the liquid has reduced to about 2 tablespoons. Cool and shred.
While the chicken cooks, prepare the poblano sauce by adding the chicken stock, poblano, garlic, jalapeño (if using), and lime to a vitamix blender. Blend on high for 4-5 minutes, or until steam starts to escape from the lid. Add the sour cream and cilantro; blend another 1 to 2 minutes. Season to taste with salt. (You can also use a different blender or food processor to blend the ingredients until smooth, then heat the sauce in a pan.)
Pour about 1/2 cup of poblano sauce into the bottom of a baking pan.
Divide the chicken between the tortillas and roll; place into the prepared pan. Pour the remaining sauce over the enchiladas. Top with cheese.
Bake for 10 minutes or until cheese is melted. Top with finely diced red onion and jalapeno, if desired.