Remove beef from refrigerator 30 minutes before cooking. Preheat oven to 400 degrees. Rub 1 teaspoon oil over beef, and season with salt and pepper. Heat 2 teaspoons oil in a large ovenproof skillet over medium-high heat. Sear beef on all sides until deep golden brown, about 4 minutes per side. Transfer skillet to oven, and cook until beef reaches desired doneness (135 degrees for medium-rare, about 15 minutes). Transfer to a cutting board, and let rest for 10 minutes. Thinly slice.
Heat remaining 2 teaspoons oil in a pan over medium-low heat. Add shallots, and cook until soft, about 4 minutes. Add sultanas, port, vinegar, chile, and thyme sprig. Raise heat to medium-high, and simmer until thickened, about 8 minutes. Let cool slightly. Pulse one-quarter of the relish in a food processor until almost smooth. Return to pan, and stir in chopped thyme, celery, salt, and pepper. (Relish can be refrigerated in an airtight container for up to 2 days; bring to room temperature before serving.)
To serve, combine mayonnaise and mustard, and spread about 2 teaspoons on each slice of baguette. Top with a slice of beef, season with salt, and top with 1 teaspoon sultana-port relish. Garnish with celery leaves.