Prepare all ingredients as listed.
In a bowl, whisk together the garlic powder, dried oregano, lemon juice, and olive oil until well combined, then season with salt and black pepper to taste.
Place the chicken breast into the marinade and coat thoroughly on all sides, ensuring it is evenly covered.
Preheat a grill or griddle pan over medium heat.
Grill the chicken for 5–7 minutes per side, or until fully cooked and the internal temperature reaches 165°F (74°C), then remove from heat and let rest for 5 minutes before slicing or serving.
Lightly grill or toast the brioche buns until golden brown, then set aside.
Assemble the sandwich by spreading mayonnaise on the inside of each bun, then layering butter lettuce, tomato slices, grilled chicken, Swiss cheese, and red onion.
Combine the sparkling water, lemon slices, and chia seeds, stirring thoroughly, then let sit for 2–3 minutes before stirring again to prevent clumping.
Serve the sandwich with carrot sticks, hummus, fresh blueberries, and the lemon chia sparkling water.