Put poblanos on a cookie sheet and broil until black. Turn and blacken on all sides. Remove poblanos and put right into a plastic bag and seal so no steam escapes. Set aside.
Meanwhile, heat oil in large skillet on medium high heat.
Add onions and cook until wilted and slightly browned.
Peel and cube cooked potatoes (about 1/2-inch cubes).
Turn skillet temp up to high and add potaotes.
You may need to add more oil. Potatoes tend to suck it all up and you want the potatoes to kind of "fry". You want to get some color on the potatoes, so stir and then let them cook and brown before stirring again. The potatoes will fall apart and kind of mash, but that's OK.
Add lots of salt and some pepper to taste--do taste to make sure it is salty enough.
Under running water, peel skins off of poblanos and remove stems and seeds.
Coarsely chop poblanos.
When potatoes have some nice color on them, add poblanos, garlic and queso fresco. Cook for another 5 minutes.
Note: I use this as a filling for vegetarian tacos or for quesadillas, but they would be good as even a side dish or for breakfast instead of hashbrowns.