1. Prepare beef: Preheat oven to 425 degrees F.
2. Stir together mustards, brown sugar, pepper, and lemon zest.
3. 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.
4. Transfer to an oiled shallow baking pan and coat with mustard mixture.
5. 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.
6. 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.
7. 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.
8. Cut beef into thin slices.
9. Add onion slices, beef, and enough dressing to arugula and beans to just coat and toss to combine well.

Yield: 2 servings