Preheat your grill to medium-high heat and season the beef filet with salt and pepper.
Place the beef filet on the grill and cook until your desired level of doneness is achieved.
While the filet is grilling, heat olive oil in a skillet over medium heat.
Add the sliced red onion and sauté until they become caramelized, stirring occasionally.
Cut the focaccia slices in half and spread butter lightly on each piece.
Toast them on a hot skillet or in the oven until golden brown.
Once cooked, let the beef rest for at least five minutes before slicing it thinly.
Build your sandwich by layering slices of beef filet, provolone cheese, caramelized onions, and aioli on the toasted focaccia.
Assemble the sandwiches and place them back on the grill or in the oven for a few minutes if you like a melted cheese effect.
Serve promptly while warm.