Preheat a grill or griddle pan on medium heat.
In a bowl, whisk together the garlic powder, dried oregano, lemon juice, and olive oil to create the marinade. Season to taste with salt and black pepper. (The marinade should taste flavorful on its own—adjust seasonings as needed.) Set aside about half of the marinade to brush on the buns.
To prepare the chicken, brush one side with the marinade and season with salt and pepper.
Place the brushed side of the chicken on the grill or griddle pan. Brush the top with more marinade. Grill until nicely marked, about 6 minutes.
Flip the chicken and brush with the remaining marinade. Close the grill (or cover the pan with a lid) and cook for another 6 minutes, or until the chicken reaches an internal temperature of 165°F (74°C). Remove and let rest for 5 minutes.
Brush the inside of each brioche bun with the reserved marinade. Lightly grill or toast until golden brown, then set aside.
Assemble the sandwich: Spread half of the mayonnaise on each bun half. Layer the bottom half with butter lettuce, tomato slices, grilled chicken, Swiss cheese, and red onion. Top with the other half of the bun.
Prepare the chia fresca, begin by squeezing fresh lemon juice and set it aside. Measure the maple syrup or honey, chia seeds, and a pinch of salt.
In a medium pitcher, combine the lemon juice with maple syrup or honey and stir well. Add the chia seeds and salt, then slowly pour in the water while stirring gently to combine. Let the mixture sit for a few minutes so the chia seeds can hydrate. Add mint.
Serve the sandwiches with a side of carrot sticks, hummus, pickle spears, along with a refreshing glass of chia fresca.