Slow Roast Beef

Ingredients

• 1 (4-lb.) New York strip roast, preferably prime, untied
• Kosher salt, freshly ground pepper
• 6 garlic cloves, finely grated
• 3 Tbsp. finely chopped rosemary
• 5 Tbsp. extra-virgin olive oil, divided
• 1/2 cup plain whole-milk Greek yogurt
• 1/2 cup sour cream
• 2 Tbsp. prepared horseradish
• 1/2 tsp. finely grated lemon zest
• Warm rolls and cornichons (for serving)