Preheat oven to 350 degrees F. Preheat a grill to medium-high heat.

Brush the slices of focaccia with the olive oil. Place on a sheet pan and grill until crispy, about 5 minutes. Set aside.

In a medium saucepan, combine the onions, wine, vinegar, brown sugar, and honey. Stir constantly over medium heat until the sugar is dissolved, about 5 minutes. Reduce heat to low and simmer for 30 minutes, until the sauce is the consistency of marmalade.

Grill the beef tenderloin, turning to mark all sides, until medium rare, about 10 to 15 minutes total. Let rest for 10 minutes, then thinly slice the meat. Place a slice of beef on each rosemary focaccia crostini and top each with a teaspoon of onion marmalade.