- For beef
- 1/4 cup coarse-grain mustard
- 3/4 teaspoon dry mustard
- 2 1/4 teaspoons dark brown sugar
- 3/4 teaspoon coarsely ground black pepper
- 1/2 teaspoon finely grated fresh lemon zest
- 1 pound trimmed beef tenderloin roast, tied
- 1 tablespoon vegetable oil
- For Salad
- 1 small red onion, thinly sliced
- 1/2 pound wax or green beans, cut diagonally into 2-inch pieces
- 4 ounces baby arugula
- For dressing
- 1 1/2 tablespoons balsamic vinegar
- 1 tablespoon fresh lemon juice
- 1/4 teaspoon Dijon mustard
- 1/4 cup olive oil
- Prepare beef: Preheat oven to 425 degrees F.
- Stir together mustards, brown sugar, pepper, and lemon zest.
- Pat beef dry and season generously with salt. Heat oil in a heavy skillet over moderately high heat until just beginning to smoke, then brown beef on all sides, about 2 minutes total.
- Transfer to an oiled shallow baking pan and coat with mustard mixture.
- Roast beef in middle of oven until a thermometer inserted 2 inches into thickest part of meat registers 125 degrees F for medium-rare, 15 to 20 minutes. Transfer to a cutting board and let stand until cool, at least 45 minutes.
- Prepare salad while beef is roasting: Chill onion slices in water to cover 30 minutes. Cook beans in boiling salted water until crisp-tender, about 5 minutes. Drain beans in a colander and rinse under cold water to stop cooking, then transfer to a large bowl. Add arugula to beans. Drain onion slices and pat dry.
- Make dressing and toss salad: Whisk together vinegar, lemon juice, mustard, and salt and pepper to taste. Add oil in a slow stream, whisking until emulsified.
- Cut beef into thin slices.
- Add onion slices, beef, and enough dressing to arugula and beans to just coat and toss to combine well.