Open-Faced Roast Beef Sandwich with Horseradish Cream and Pickled Onions

**Prepare the Pickled Red Onions:** In a small saucepan, combine white vinegar, water, sugar, and salt. Bring to a simmer over medium heat, stirring until sugar and salt are dissolved. Remove from heat.

Place the thinly sliced red onion in a heat-safe jar or bowl. Pour the vinegar mixture over the onions, making sure they are fully submerged. Let cool to room temperature, then refrigerate for at least 30 minutes (or up to a week). The longer they sit, the more pickled they will be.

**Make the Horseradish Cream Sauce:** In a medium mixing bowl, whip the heavy cream until stiff peaks form. Gently fold in the prepared horseradish and sour cream (if using). Season with salt and pepper to taste. Adjust the amount of horseradish to your desired level of spiciness. Refrigerate until ready to use.

**Toast the Rye Bread:** Toast the rye bread slices to your liking using a toaster or broiler.

**Assemble the Sandwiches:** Spread a generous amount of horseradish cream sauce on each slice of toasted rye bread. Top with thinly sliced roast beef and a spoonful of pickled red onions.

Garnish with fresh parsley and serve immediately.