1 lb/455 g green beans, ends trimmed
1 tbsp canola, grapeseed, or peanut oil
2 shallots, halved and sliced paper thin
1 tbsp hot sweet mustard
1 tbsp low-sodium soy sauce
1 tbsp sherry or balsamic vinegar
Kosher salt and freshly ground black pepper
1 tbsp toasted sesame seeds (optional)
Bring a pot of heavily salted water to a boil over high heat. Fill a large bowl halfway with ice water and set aside.
Add the green beans to the boiling water and cook until bright and knife tender, 2 to 3 minutes. Drain and place in the ice water bath until cool. Drain, pat dry, and set aside until ready to use.
Heat the oil in a medium pan over medium-high heat. When it shimmers, add the shallots and cook until golden brown, about 3 minutes. Add the green beans and cook until warmed through and golden brown, about 5 minutes.
Add the mustard, soy sauce, and vinegar and cook, stirring frequently, until the sauce just coats the bottom of the pan. Season with salt and pepper, scatter with sesame seeds, if using, and serve.