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.

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, combine all ingredients together. Stir well. Let sit 5 minutes for chia to bloom.