Prepare the marinade: combine the olive oil, red wine vinegar, soy sauce, and garlic. Trim and discard any fat from the steak and place it in a resealable plastic bag along with the marinade. Refrigerate for at least 4 hours or preferably overnight.
Whisk together the sour cream, dijon mustard, horseradish and salt. Set aside.
Preheat the oven to 350 degrees F. Thinly slice the baguette on a slight bias and brush with 1 tablespoon olive oil. Sprinkle with salt. Bake for 10-15 minutes, or until crunchy. Set aside.
Heat 2 tablespoons olive oil in a large skillet on low heat. Add onions and a pinch of salt. Have a measuring cup full of water nearby. Cook the onions on low heat, stirring frequently, until the onions begin to brown. Turn up the heat to medium and continue caramelizing the onions. As a glaze develops on the bottom of the pan, add a few tablespoons of water and scrape up the brown bits, incorporating them into the onions. The heat will cause the water to evaporate. Repeat this process for approximately 25-30 minutes, keeping a close eye on the onions to make sure they don't burn. When the onions have turned a dark brown color and are both bitter and sweet in flavor, remove from the heat and set aside.
Preheat the broiler and place a rack toward the top of the oven. Remove steak from the marinade and pat dry with paper towels. Season generously with salt and pepper then broil for 5-6 minutes per side. Allow the meat to rest for at least 10 minutes, then slice very thin against the grain. Allow some of the juices to drain on paper towels.
Slice the serrano peppers thinly.
Spread a thin layer of onion confit on each crostini. Top with a slice of flank steak followed by a dollop of horseradish cream. Place a thin slice of pepper on top. These can be made up to 2 hours before serving.