Prep all ingredients as listed above.
Preheat a grill or griddle pan over medium heat.
In a bowl, whisk together the garlic powder, oregano, lemon juice, and olive oil for the marinade. Season with salt and pepper to taste. Set aside half the marinade for brushing on 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 with more marinade, and grill for another 6 minutes. Cover the grill or pan and cook the chicken to an internal temperature of 160°F (71°C). Let rest for 5 minutes.
Brush the inside of each brioche bun with the reserved marinade and lightly grill or toast until golden brown. Set aside.
Assemble the sandwiches by spreading mayonnaise on both sides of each bun. Layer with butter lettuce, tomato, grilled chicken, Swiss cheese, and red onion.
To make the chia fresca, combine all the ingredients together. Let sit for 5 minutes for the chia to bloom.
Serve the sandwiches with carrots and hummus, blueberries, and chia fresca.