In a large skillet over medium-high heat, warm the butter and olive oil. Add the onions and salt. Cook until they become tender and have reduced in volume by half, about 10 minutes, stirring frequently to prevent burning. Stir in the vinegar, sugar, and currants. Reduce the heat to medium-low and cook until the onions become jam-like and most of the liquid has evaporated, about 35 minutes. Remove from the heat and let cool to room temperature. The jam will thicken as it cools. (The jam may be made up to 2 days ahead. Cover and refrigerate. Bring to room temperature before serving.)
Prepare the grill for direct cooking over medium heat (350° to 450°F).
Lightly brush the steaks on both sides with the oil and season with the salt and pepper. Allow the steaks to stand at room temperature for 15 to 30 minutes before grilling.
Brush the cooking grates clean. Lightly brush one side of the baguette slices with oil and grill them over direct medium heat until toasted, 1 to 2 minutes (grill one side only). Allow to cool, and then rub the grilled sides lightly with the garlic.
In a small bowl combine the cream ingredients. Cover and refrigerate until ready to use.
Grill the steaks over direct medium heat, with the lid closed as much as possible, until cooked to your desired doneness, 11 to 13 minutes for medium rare, turning once or twice (if flare-ups occur, move the steaks temporarily over indirect heat). Remove from the grill and let rest for 3 to 5 minutes. Cut the steaks into ¼-inch slices.
Spread a layer of jam on each piece of toasted baguette (crostini). Place a slice of meat on the jam and top with a small dollop of the horseradish cream. Sprinkle with chopped fresh chives, if desired.