Set up Nomiku water bath and turn temperature to 79ºC (174.2ºF).
Using a filet knife, trim the fat, papillary muscle, valves, and connective tissues from the beef heart. The goal is to end up with beautiful lean meat. Cube the cleaned heart.
Put the cubed heart in a bag and add extra virgin olive oil, kosher salt, and rosemary. Seal with a vacuum sealer or with the water displacement method using a freezer-safe zip bag. Sous vide for 24 hours.
Move frozen phyllo dough to the refrigerator to defrost.
An hour before serving, prepare phyllo crust and filling. Pre-heat oven to 350ºF. Melt half a stick of butter in a small bowl for the crust. Get two 7” springform pans (or 4 smaller ramekins). With a brush, start buttering each separate sheet of phyllo and cover the bottom and sides of the pans. It’s ok to scrunch them for a more rustic look — phyllo is very forgiving. Repeat 5x. Bake in oven for 15-20 minutes or until brown.
Remove beef heart from water bath, remove rosemary, and strain the heart jus. There should be 2 cups of jus, if not, add water. Whisk 3 tsp Wondra flour into the jus.
In a cast iron over medium-high heat, melt the other ½ stick of butter. Brown the pearl onions and mushrooms. Add in carrots and peas and warm through. Pour in the heart jus and tomato paste. Stir until everything is coated. Turn off heat.
Once the heat is off, mix in cubed heart until it is fully incorporated. Season with Omnivore Salt to taste.
Spoon into phyllo crust and serve immediately.
If you’d like to include some little phyllo dough hearts like we did, you can buy a heart mold from Daiso!