Heat a cast-iron skillet to med-high heat. Add both types of oil to the pan and when pan is hot, add the shishito peppers. Stir them around so they all have some oil. Season liberally with salt and pepper. Once you see that they are starting to blister, move them around frequently so they don't burn.
Once the majority of the peppers have blistered, reduce the heat to medium and add the red pepper flakes and garlic to the pan. The pan will be really hot, so keep stirring everything around so the garlic does not burn. When the garlic has softened and has cooked, they are done. Sprinkle sesame seeds over the peppers and eat immediately, or store in the fridge for a couple days.