In a large pot over high heat, bring 6 cups of water and 2 tablespoons of kosher salt to a boil.
Thoroughly wash 1 pound of snap peas and 1 orange (including the rind) under cool, running, drinkable water. Set the orange aside.
Pinch off the top stem and pull away the string that runs along the straight side of the snap pea. Note: Not all of them have strings, so if you’re lucky, you won’t find any!
Add snap peas to the boiling water and submerge below the waterline. Cook until bright green and crisp-tender, testing after 2 minutes. Remove with a wire strainer or drain into a colander set in the sink. Run under cold water until they are cool to the touch.
In a large sauté pan over medium heat, melt 2 tablespoons of unsalted butter. Add 3 tablespoons of sesame seeds and zest from half the orange. Cook until the sesame seeds just start to turn golden, about 2 minutes.
Add the cooked snap peas and 1 tablespoon of toasted sesame seed oil. Cook until warmed through. Remove from heat and add 1 teaspoon of rice vinegar. Season with a generous pinch of flake salt.