- 5tablespoons olive oil
- 3/4pound mixed mushrooms (such as button, cremini, and shiitake), stems trimmed and cut into bite-size pieces
- kosher salt and black pepper
- 1/4cup apple cider vinegar
- 2teaspoons Dijon mustard
- pinch sugar
- 1 small head frisée, leaves torn (about 4 cups)
- 1/2 head radicchio, leaves torn (about 2 cups)
- 4 6-ounce pieces skinless haddock, cod, or striped bass fillet
- Heat 2 tablespoons of the oil in a large skillet over medium-high heat. Add the mushrooms and ¼ teaspoon each salt and pepper and cook, tossing frequently, until golden and tender, 10 to 12 minutes. With a slotted spoon, transfer to a plate.
- Remove the skillet from heat. Add the vinegar and scrape up any browned bits. Whisk in the mustard, sugar, and ¼ teaspoon each salt and pepper; gradually whisk in 2 tablespoons of the remaining oil. Add the frisée, radicchio, and mushrooms and toss to coat.
- Meanwhile, season the fish with ¼ teaspoon each salt and pepper. Heat the remaining tablespoon of oil in a large nonstick skillet over medium-high heat. Cook the fish until opaque throughout, 3 to 4 minutes per side. Serve with the salad.