- Preheat oven to 400°F.
- Combine peppercorns, fennel seeds, and 1/2 teaspoon of salt in small zip-close plastic bag; press out air and seal bag. With meat mallet or rolling pin, pound spices until crushed. Spread spice mixture on sheet of wax paper. Roll tenderloin in spice mixture until coated, pressing so it adheres.
- Heat oil in small heavy flameproof roasting pan over medium-high heat. Add beef and cook until browned on all sides, about 8 minutes.
- Transfer roasting pan to oven and roast until instant-read thermometer inserted into center of beef registers 145°F, about 30 minutes. Transfer beef to cutting board and let stand 10 minutes.
- Meanwhile, to make sauce, spray roasting pan with nonstick spray and set over medium heat. Add onion and cook, stirring, until softened, about 5 minutes. Add wine and cook, stirring, 2 minutes. Whisk in flour and cook, stirring, about 1 minute. Whisk in broth, mustard, and remaining 1/4 teaspoon salt; bring to boil. Cook, stirring, until sauce bubbles and thickens, about 4 minutes longer. Remove pan from heat; swirl in butter until melted. Cut beef against grain into 12 slices and serve with sauce.
- Per serving:3 slices beef and 2 tablespoons sauce
A spice grinder or mini food processor is the fastest way to grind spices, although a coffee grinder also works well. Just be sure to wipe the inside of the grinder clean with a clean cloth dipped into white (distilled) vinegar, then thoroughly dry.