2 tablespoons extra-virgin olive oil
1 pound precooked diced red-skinned potatoes, (about 2 1/2 cups)
2 tablespoons whole-grain mustard
2 tablespoons cider vinegar
salt, or to taste
1/8 teaspoon freshly ground pepper
4 ounces smoked trout, skin removed, flaked
4 cups thinly sliced mustard greens
Add potatoes and cook, stirring occasionally, until golden and crispy, adjusting heat if necessary to prevent burning, 8 to 12 minutes.
Add to the potatoes along with trout and greens.
Cook, stirring, until the greens are just wilted, 30 seconds to 1 minute.
Remove from heat; stir 1 to 2 tablespoons water into the hash if it seems dry.