Preheat the oven broiler.

Heat the 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. If reheating from raw or cold, cook the beef to an internal temperature of 145°F (63°C) for medium-rare or 160°F (71°C) for medium.

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 the sautéed onions and mushrooms. Close with the top half of the bun.

To make the chia fresca, combine all ingredients together. Let sit 5 minutes for chia to bloom.

Serve the sandwich with carrot sticks, hummus, pickle spear and sparkling chia fresca.