Place 1 poblano pepper on the stove per burner. This way they will have enough space to char evenly. Set the heat to medium and let them char for 4-5 minutes on each side or until blackened all over. Remove them from the heat.
Place the poblanos in a plastic bag and seal it or tie it closed. Let them steam in the bag for 15 to 20 minutes. The longer, the better but don't let them cool fully either.
Remove them from the bag and use your thumbs to carefully rub against the blackened poblano skin to peel them under cold running water. Remove as much of the blackened bits as possible. Don't worry if the peppers break a little bit.
Place the peeled poblanos on a cutting board. Use a knife to make a lengthwise slit down the center. Remove the seeds and stems from the poblanos. Then slice them into 1/2" thick strips. You can slice them thinner or thicker too, it's up to you.
Heat a large pan over medium heat. Add the vegetable oil and let it heat for another minute. Add the onion and let it cool for 5-6 minutes or until soft and fragrant. Don't forget to stir occasionally. Don't worry if the onion browns a little bit.
Add the sliced poblanos and yellow corn into the pan. Stir to combine and let everything cook for another 3-4 minutes.
Add the milk and cheese to the poblano mixture and stir well. Season with salt and pepper to taste.
Stir occasionally. Allow the mixture to simmer for 15-20 minutes or until the milk has thickened slightly and the cheese has melted. It shouldn't be too liquid-y.
Remove from the heat and serve immediately so that the cheese is still melty and the sauce doesn't seize.
Storage: Once fully cooled, refrigerate leftover rajas in an airtight container for up to 5 days. Throw them out immediately if they taste bitter or sour.Reheat: To reheat, pop them into the microwave for up to a minute or until warm. You can also heat them in a pan, stirring occasionally, for 5-7 minutes over medium-low heat.