Prep all ingredients as listed above.
Preheat a grill or griddle pan over medium heat.
In a bowl, whisk together garlic powder, oregano, lemon juice, and olive oil for the marinade. Season with salt and pepper to taste. Set aside half of the marinade for brushing the buns.
Brush one side of the chicken breast with the remaining marinade and season with salt and pepper.
Place the marinated side of the chicken on the grill or griddle pan. Brush the top with more marinade. Grill for about 6 minutes, then flip, brush again, and grill for another 6 minutes. Cover the grill or pan and cook until the chicken reaches an internal temperature of 165°F (74°C). Let rest for 5 minutes before slicing or serving.
Brush the inside of each brioche bun with the reserved marinade and lightly grill or toast until golden brown.
Assemble each sandwich by spreading mayonnaise on both bun halves. Layer with butter lettuce, tomato, grilled chicken, Swiss cheese, and red onion.
For the chia fresca, combine all the ingredients. Let sit for 5 minutes to allow the chia seeds to bloom.
Serve the sandwich with carrots, hummus, fresh blueberries, and sparkling chia fresca.