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. Remove from the pan and set aside.

Add the roast beef to the same pan and sauté until heated through and lightly browned, about 3–4 minutes. Cook the beef to an internal temperature of 145°F (63°C) for medium-rare or 160°F (71°C) for medium, if reheating from raw or cold.

Place the provolone cheese over the roast beef in the pan. Transfer the pan (or use 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.

To make the chia fresca, squeeze fresh lemon juice and set aside. Measure the maple syrup or honey, chia seeds, and a pinch of salt. In a medium pitcher, combine the lemon juice and maple syrup or honey. Stir well. Add the chia seeds and salt, then slowly pour in sparkling water while stirring gently to combine. Pour the chia fresca into glasses and garnish each with a sprig of fresh mint.

Plate each sandwich with carrot sticks, a side of hummus, and a pickle spear. Serve with sparkling chia fresca on the side.