Preheat the oven broiler.

Heat olive oil in a pan over medium heat. Sauté the sliced onion and mushrooms until golden brown and tender, about 5 minutes. Set aside.

Add the roast beef to the same pan and sauté until heated through and lightly browned, about 3–4 minutes.

Place provolone cheese over the roast beef. Transfer the pan (or an oven-safe dish) to the broiler for 1–2 minutes, or until the cheese is melted and bubbly.

Toast the brioche buns in a toaster or under the broiler until golden brown.

Spread herb mayo inside each toasted brioche bun.

Assemble the Sandwich: Layer the roast beef with melted provolone, then top with sautéed onions and mushrooms. Close with the top half of the bun.

Add lemon slices and ice cubes to the sparkling water, and stir to combine.

Plate the sandwiches with carrot sticks and a side of hummus. Serve with sparkling lemon water.