Remove the bison fillet from the refrigerator 2-3 hours before cooking to bring it to room temperature. Rinse under cold running water and pat dry with paper towels.
Cut the onion into rough rings and slice the buffalo mozzarella diagonally into thin slices. Preheat your oven to 90°C (194°F).
Heat the oil or clarified butter in a large pan over medium-high heat for 2-3 minutes until very hot. Carefully place the bison in the pan and sear on all sides for 3-4 minutes per side, developing a brown crust without charring.
Remove the seared bison from the pan and pat away excess oil with paper towels. Season all sides with salt and pepper from the mill.
Arrange the onion rings on the bottom of a roasting pan. Place the seared bison on top and arrange the mozzarella slices over the meat.
Insert a meat thermometer into the thickest part of the bison, making sure the tip is in the center of the meat. Place the pan in the preheated 90°C (194°F) oven and roast for 90-120 minutes, until the internal temperature reaches 60°C (140°F).
Remove the roast from the oven and let it rest for 10 minutes on a cutting board before slicing and serving. The meat will continue to cook slightly during resting, reaching approximately 62-63°C (145°F).
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