Simmer canned tomatoes with a halved onion and beef tallow.
Nestle the chuck roast with the tomatoes and onion.
Add fennel to the mixture.
Braise the meat for 2 to 2 ½ hours in the oven.
Blend the fennel and onion into the sauce using an immersion blender or remove and discard them and whisk the beef fat into the tomatoes.
