Prepare all ingredients as listed. In a bowl, whisk together the garlic powder, dried oregano, lemon juice, and olive oil until well combined. Season with salt and black pepper to taste, adjusting until the marinade is well balanced.
Place the chicken breast in the marinade and coat thoroughly on all sides.
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). 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. Stir thoroughly, let sit for 2–3 minutes, then stir again to prevent clumping.
Serve the sandwich with carrot sticks, hummus, blueberries and lemon chia sparkling water.