Preheat oven to 350 degrees F.

Preheat a grill to medium-high heat.

Thinly slice the rosemary focaccia bread.

Brush the slices of focaccia with olive oil.

Place the focaccia slices on a sheet pan.

Grill until crispy, about 5 minutes.

Set aside the grilled focaccia.

In a medium saucepan, combine the thinly sliced onions, red wine, red 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 the beef tenderloin rest for 10 minutes.

Thinly slice the rested beef tenderloin.

Place a slice of beef on each rosemary focaccia crostini.

Top each crostini with a teaspoon of onion marmalade.

Serve immediately.