How to Make It
Preheat oven to 400°F. Rub beef all over with 1 tablespoon of the pepper and 2 teaspoons of the salt. Heat 1 tablespoon of the oil in a large cast-iron skillet over medium-high until shimmering. Add beef; cook, turning occasionally, until browned all over, 6 to 8 minutes. Transfer skillet to oven. Roast until a thermometer in thickest portion of beef registers 115°F, 12 to 15 minutes. (Do not turn oven off.) Transfer beef to a cutting board; rest 20 minutes.
Meanwhile, brush both sides of baguette slices with 3 tablespoons of the oil. Arrange slices in a single layer on a baking sheet; sprinkle with 1/4 teaspoon of the salt. Bake until lightly browned, 8 to 12 minutes. Cool 10 minutes.
Stir together celery, 1 tablespoon of the vinegar, 1/4 teaspoon of the salt, and remaining 1 tablespoon oil in a small bowl. Let stand 10 minutes. Stir together sour cream, garlic, 2 ounces of the blue cheese, and remaining 1/2 tablespoon vinegar, 1/2 teaspoon salt, and 1/4 teaspoon pepper in a medium bowl.
Thinly cut beef into 1/4-inch-thick slices. Spread about 1 1/2 teaspoons blue cheese mixture on each baguette slice. Top evenly with beef slices and celery mixture. Sprinkle with remaining 1 ounce blue cheese.