• Origin: The idea of serving sliced roast beef on bread is strongly tied to English and American traditions. In the U.K., roast beef has been a Sunday dinner centerpiece for centuries. By the late 1800s and early 1900s, U.S. delis (especially Jewish delis) popularized sliced roast beef sandwiches, sometimes hot (with gravy) and sometimes cold.
• Famous versions: The “Beef on Weck” (Buffalo, NY) and Arby’s chain (founded in 1964) are iconic examples.

 Lettuce & Tomato

• These became classic American sandwich toppings in the early 20th century, especially after refrigeration and year-round produce distribution became common. They added freshness and crunch, turning heavy meat sandwiches into a more balanced bite.
• The BLT (bacon, lettuce, tomato) sandwich helped cement this trio as the go-to garnishes.

 Swiss Cheese

• “Swiss” in the U.S. usually refers to Emmental-style cheese from Switzerland, but mass-produced versions became widely available in America in the mid-20th century.
• Deli sandwiches began pairing mild Swiss with meats like turkey, ham, or roast beef for its nutty flavor and easy melt.

 Bacon

• Bacon on sandwiches took off in the 1950s–60s diner culture in the U.S., though the BLT already made it popular earlier. Adding it to roast beef or turkey sandwiches became a way to “supercharge” them with salty, smoky crunch.

 Crispy Onions

• Crispy fried onions as a topping are an American innovation, going mainstream in the mid-1900s. Originally popularized with green bean casserole (1955, Campbell’s Soup recipe), they later appeared in burgers and deli sandwiches for texture.

 Pane Rustico

• Pane Rustico is an Italian rustic bread — airy, crusty, and chewy. It reflects the influence of Italian bakery culture in America, especially in the Northeast. By the late 20th century, it became common to see ciabatta, pane Rustico, and focaccia used instead of plain white sandwich bread for “gourmet deli” sandwiches.

 Putting It All Together

The exact sandwich you listed doesn’t have a single historic birthplace — it’s a fusion of classic American deli traditions with Italian bread culture. Likely, this kind of layered roast beef sandwich emerged in the 1980s–2000s gourmet deli and café boom, when chefs started upgrading everyday sandwiches with artisan bread, specialty cheeses, and toppings like crispy onions.

It’s essentially:

• Roast beef (English/American deli)
• BLT elements (lettuce, tomato, bacon)
• Swiss cheese (European turned American deli staple)
• Crispy onions (American diner/comfort food)
• Pane Rustico (Italian bread influence)