In a bowl, toss the peppers with the avocado oil or olive oil.
Heat a large heavy skillet (preferably cast iron) over high heat until very hot. Add the shishito peppers. Spread out into one layer and cook until completely browned and blistered on the underside, about 2 minutes. Flip the peppers and cook on the other side, another 2-3 minutes. If you prefer the peppers a bit softer, lower heat to medium and cook another minute or two.
Remove the peppers from the heat and sprinkle with flaky salt (and lime juice if using). Serve immediately.