Open-Faced Rye Sandwich with Roast Beef, Pickled Onions & Creamy Mustard Sauce

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

Place the thinly sliced red onion in a heat-safe bowl. Pour the hot vinegar mixture over the onions, making sure they are submerged. Let sit for at least 15 minutes, or up to an hour, stirring occasionally. The longer they sit, the more pickled they will become. Drain well before using.

**Make the Creamy Mustard Sauce:** In a small bowl, whisk together the mayonnaise, Dijon mustard, and horseradish until smooth. Taste and adjust seasonings as needed. If you like a bit more heat, add a pinch of cayenne pepper.

**Prepare the Rye Bread:** Preheat oven to 350°F (175°C). Slice the rye bread loaf into 1-inch thick slices. Arrange the slices on a baking sheet.

Toast the rye bread slices in the preheated oven for 5-7 minutes, or until lightly golden and crisp around the edges. Watch carefully to prevent burning.

**Assemble the Open-Faced Sandwiches:** Spread a generous amount of the creamy mustard sauce onto each toasted rye bread slice.

Top with thinly sliced roast beef, ensuring it's evenly distributed.

Spoon the pickled red onions over the roast beef.

Garnish with fresh chopped dill (optional).

Serve immediately. Enjoy!