The beef is especially delicious when grilled, but it can also be seared in a pan and oven roasted.
Whisk together vinegar, lemon juice, red wine, oil, rosemary, garlic, salt, and peppercorns in a large bowl. Add fillet, turning to coat well. Cover with plastic wrap; marinate overnight in refrigerator, turning meat several times.
Heat a grill until coals are hot. Grill fillet until medium rare, 145 degrees on a meat thermometer, about 30 minutes. If roasting in oven, heat to 450 degrees. Heat a large iron skillet over low heat until very hot (a few drops of water splashed in pan should evaporate almost immediately). Place fillet in pan and cook until brown on all sides, 5 to 10 minutes. Place skillet in oven; roast until meat is medium rare, about 20 minutes. Remove from pan; let fillet cool to room temperature.
Cut bread on a slight angle into 3/8-inch-thick slices; arrange on a baking sheet. Melt butter and brush each slice lightly. Toast in a 350 degrees oven until golden brown. Set aside. Combine creme fraiche and horseradish. Set aside.
To assemble hors d'oeuvres, slice fillet into 1/4-inch-thick slices. Spread a little horseradish sauce on toast and top with a slice of beef. Garnish with Red Onion Confit and additional rosemary.