Give the poblanos a good rinse and then roast them in a 400F oven for 20-30 minutes. I usually flip them over halfway through the roasting period.
Finely cube a potato and add it to salted, boiling water in a saucepan over high heat. Cook for 4-6 minutes or until the potatoes are 'al dente', still slightly firm and not quite cooked all the way through.
When the poblanos are done roasting let them cool down for a few minutes and then remove as much skin as you can. De-stem and de-seed the poblanos. Cut them lengthwise into strips and then cut the strips in half; you'll end up with bite-sized poblano strips.
Thinly slice an onion and saute it in a tablespoon of olive oil over medium heat. I also added a tablespoon of butter but this is optional.
Cook the onion until soft and just starting to brown, approx. 7-8 minutes. Add 2 minced cloves of garlic and give the garlic a brief saute, 30 seconds or so. Add the poblano strips and cook briefly until everything is at a uniform temp.
Add 1 teaspoon of salt, some freshly cracked black pepper, 1/2 cup heavy cream and 1/2 cup of stock. (Note: you can skip the stock and use all cream if you want, anywhere from 3/4 cup to 1 cup of cream will do the job.)
Once the liquid is simmering, add the parboiled potatoes. Combine well and simmer for another 5 minutes or so.
Take a final taste for seasoning. I added another generous pinch of salt to this batch.
Serve immediately as is or you can create some on-the-fly tacos. For tacos, simply add the Rajas to warm corn tortillas and top with Cotija cheese.