Rinse and thoroughly dry the peppers with a clean kitchen towel to prevent splattering.
Turn your exhaust fan on to its highest setting and, if possible, open a window (cooking shishitos can get a bit smoky!).
Heat a large (preferably cast iron) skillet over medium-high heat. Once hot, add the oil and peppers and sprinkle with a healthy pinch of sea salt (work in batches if using a smaller pan or cooking a larger amount of peppers). Cook the peppers in a single layer (it’s okay if a few aren’t touching the bottom of the pan), undisturbed, until charred in places — about 2-3 minutes. Then toss and continue to cook, tossing occasionally, until charred on all sides and shriveled, about 6-7 minutes more. Transfer peppers to a bowl to cool.
Return the skillet to the heat and add the green onions. Cook, turning occasionally, until charred on all sides — about 5-6 minutes. Transfer charred green onions to the bowl with the peppers.
When cool enough to handle, remove the stems from the peppers and discard. Roughly chop the peppers and green onions.
Add the chopped peppers and green onions to a medium bowl along with the mango, avocado, cilantro, lime juice, and salt. Stir well. Taste and adjust as needed, adding more lime juice for brightness, maple syrup (or honey) if your mango isn’t adding enough sweetness, or salt for overall flavor.
Best when fresh. Leftover salsa will keep in a sealed container in the refrigerator for 1-2 days, but the avocado will begin to brown. Not freezer friendly.
Notes
*Nutrition information is a rough estimate calculated without optional ingredients.