Pickled red onions: In a small saucepan, combine red wine vinegar, sugar, and water. Stir and bring to a boil. Turn off the heat. Pour the vinegar mixture over the red onions in a heat resistant bowl and let sit uncovered to cool for at least 15 minutes.
Horseradish cream: In a small bowl, mix together sour cream, horseradish, kosher salt, pepper, and hot sauce.
Assembly: Slice croissants in half and place on a parchment-lined baking sheet. On the top croissant half, spread the olive tapenade and place on the Swiss cheese and marinated tomatoes. On the bottom croissant half, spread 2 tablespoons horseradish cream and top with 5 slices Castle Wood Reserve® Angus Seasoned Roast Beef. Repeat with the other croissant.
Bake open-face for 10 minutes. Remove from the oven and top each sandwich with ¼ cup pickled red onion and ½ cup (20 G) baby spinach. Close the sandwich. Serve with a skewered tomato and gherkin for garnish.