Heat oil in a large skillet over medium-high heat.
Add the jalapeño and serrano peppers and cook for about 6-8 minutes, stirring occasionally, until the peppers are charred and blistered and all sides.
Add the onions and cook for 3-5 minutes, stirring occasionally, until they have softened and started to caramelize.
Remove the skillet from the heat. Add the lime juice and soy sauce. Stir together to combine.
Transfer blistered peppers and onions to a plate or bowl and serve immediately or at room temperature.
Use a large skillet or frying pan so that the peppers are in a single layer and are not overcrowded as they cook. This will help ensure that the skins char and blister evenly.
I would recommend using a cast-iron skillet because it retains and distributes heat very well, helping the peppers char faster and cook evenly.
You won’t regret leaving the stems on the peppers. The stems will help you pick up and eat the peppers after they’ve been cooked.
If you’re worried about this dish being too spicy, feel free to use a different chiles like poblano peppers or anaheim peppers. Familiarize yourself with the Scoville Scale. Certain chile peppers may not be as intimidating as you may think!