In a bowl, whisk together soy sauce, olive oil, brown sugar, vinegar, Dijon mustard, and minced garlic. Place steak in a zip-top bag, pour in marinade, seal, and refrigerate for 4 to 24 hours.

Slice yellow onions thinly and heat olive oil in a skillet over low heat, then add onions and caramelize for 30 to 40 minutes until golden.

In a mixing bowl, combine sour cream, mayonnaise, horseradish, apple cider vinegar, and a pinch of salt and pepper; refrigerate for at least 30 minutes.

Preheat broiler, slice baguette, brush with olive oil, and broil for 2 to 3 minutes until golden brown.

Remove marinated steak, pat dry, preheat grill or skillet over high heat; cook steak for about 4 to 5 minutes per side.

Assemble crostini by placing toasted baguette slices on a platter, topping with caramelized onions, sliced steak, horseradish cream, chives, and flaky sea salt.