Preheat the oven broiler.

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

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

Place the 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 by layering the roast beef with melted provolone, then topping with sautéed onions and mushrooms. Close with the top half of the bun.

Prepare the chia fresca: combine all the ingredients. Let sit 5 minutes for chia seeds to bloom.

Plate the sandwiches with carrot sticks, hummus, and pickle spears. Serve alongside the sparkling chia fresca.