1. In a large skillet, heat butter and oil over medium heat. Add onions, and stir to coat; cook, stirring occasionally, until they begin to soften, about 10 minutes. Reduce heat to medium-low; continue cooking, stirring every 20 minutes, until onions are amber colored and completely caramelized, just over 1 hour. Remove from heat, and transfer to a bowl to cool.

2. In a small bowl, mix horseradish and cream cheese; season with salt and pepper. Spread two slices of bread with a thin layer of horseradish cream cheese; cover one with a layer of onions, and top with roast beef and other bread slice. Use a serrated knife to trim crusts and cut into four small squares. Repeat with remaining ingredients. Cover with damp paper towels until ready to serve.