Step 1
Remove roast from refrigerator 1 hour before cooking to allow it to come to room temperature.
Step 2
Preheat oven to 325 degrees F (165 degrees C).
Step 3
Rub salt and black pepper over the surface of the roast. Heat 2 tablespoons oil in a Dutch oven over medium heat. Add onions and fennel; cook and stir for 5 minutes. Add garlic; cook 1 minute more. Remove onion mixture from Dutch oven.
Step 4
Pour remaining 2 tablespoons oil into the Dutch oven over medium heat. Add roast; brown on all sides about 10 minutes. Remove roast and return onion mixture to Dutch oven. Place roast on top and cover the Dutch oven.
Step 5
Bake in the preheated oven until fork tender 2 to 3 hours.
Step 6
Remove roast from Dutch oven. Puree half the onion-fennel mixture in a food processor.
Step 7
Melt butter in a skillet over medium heat; add flour and black pepper. Cook and stir until browned 2 to 3 minutes. Whisk 1/2 cup beef stock and the onion-fennel puree into flour; add remaining 1/2 cup stock. Simmer until thickened 3 to 5 minutes. Mince reserved fennel fronds and add to gravy; season to taste with additional salt and pepper.
Step 8
Place roast on a platter; top with remaining onion-fennel mixture and half the gravy. Serve with remaining gravy on the side.