Step 1

Pat roast dry with paper towels. Place roast fat cap side down and trim off strip of meat that is loosely attached to thicker side of roast. Rotate roast 180 degrees and trim off strip of meat and fat from narrow side of roast. (After trimming, roast should be rectangular with roughly even thickness.) Cut trimmings into 1-inch pieces. Transfer trimmings to small bowl, wrap tightly in plastic wrap, and refrigerate.

Step 2

Using sharp knife, cut slits ½ inch apart and ¼ inch deep in crosshatch pattern in fat cap of roast. Sprinkle all sides of roast evenly with 2 tablespoons salt and 1 teaspoon pepper. Wrap in plastic and refrigerate for 6 to 24 hours.

Step 3

Adjust oven rack to lowest position and heat oven to 300 degrees. Trim and discard ¼ inch from end of each potato. Cut each potato in half crosswise. Toss potatoes with remaining 2 teaspoons salt and remaining 1 teaspoon pepper and set aside.

Step 4

Place oil in large roasting pan. Place roast, fat cap side down, in center of pan and scatter trimmings around roast. Cook over medium heat, stirring trimmings frequently but not moving roast, until fat cap is well browned, 8 to 12 minutes. Flip roast and continue to cook, stirring trimmings frequently, until bottom of roast is lightly browned and trimmings are rendered and crisp, 6 to 10 minutes longer. Remove pan from heat and transfer roast to plate. Using slotted spoon, transfer trimmings to medium saucepan, leaving fat in pan.

Step 5

Arrange potatoes in single layer, broad side down, in pan. Return pan to medium heat and cook, without moving potatoes, until well browned around edges, 15 to 20 minutes. (Do not flip potatoes.) Off heat, lay 18 by 22-inch sheet of aluminum foil over potatoes. Using oven mitts, crimp edges of foil to rim of pan. With paring knife, poke 5 holes in center of foil. Lay roast, fat side up, in center of foil. Transfer pan to oven and cook until meat registers 115 degrees, 1 to 1¼ hours.

Step 6

While roast cooks, add broth, thyme sprigs, rosemary sprigs, gelatin, and garlic to saucepan with trimmings. Bring to boil over medium-high heat. Reduce heat and simmer for 15 minutes. Strain mixture through fine-mesh strainer into 4-cup liquid measuring cup, pressing on solids to extract as much liquid as possible; discard solids. (You should have 4 cups liquid; if necessary, add water to equal 4 cups.)

Step 7

When meat registers 115 degrees, remove pan from oven and increase oven temperature to 500 degrees. Transfer roast to carving board. Remove foil and use to tent roast. Using offset spatula, carefully flip potatoes. Pour strained liquid around potatoes and return pan (handles will be hot) to oven (it's OK if oven has not yet reached 500 degrees). Cook until liquid is reduced by half, 20 to 30 minutes.

Step 8

Carefully transfer potatoes to serving platter. Pour liquid into fat separator and let settle for 5 minutes. Slice roast and transfer to platter with potatoes. Transfer defatted juices to small bowl. Serve, passing juices separately.