Make the sauce: Whisk together the melted butter, miso paste, garlic, scallion and 1 tablespoon water in a small bowl until combined. Set aside.
Blister the green beans: Pat the green beans dry with a paper towel. Add the canola oil to a large cast-iron skillet over medium-high heat. When the oil is hot, carefully add the green beans and sprinkle with salt. Cook until browned, about 3 minutes on each side.
Finish the dish: Add the miso butter mixture and turn the heat off. Using tongs, toss the beans with the sauce to coat, adding a tablespoon of water if necessary to loosen the sauce. Garnish with the crispy onions.