Prepare all ingredients as listed.
In a bowl, whisk together garlic powder, dried oregano, lemon juice, and olive oil. Season with salt and black pepper.
Add the chicken breast to the marinade and coat evenly.
Preheat a grill or griddle pan over medium heat.
Grill the chicken until fully cooked, reaching an internal temperature of 165°F (74°C). Remove from heat and allow to rest briefly.
Lightly grill or toast the brioche buns until golden.
Spread mayonnaise on the inside of each bun. Layer with butter lettuce, tomato, grilled chicken, Swiss cheese, and red onion.
In a glass or pitcher, combine sparkling water, lemon slices, and chia seeds. Stir thoroughly, let rest briefly, then stir again to prevent clumping.
Serve the Greek grilled chicken sandwich with carrots and hummus, blueberries, and lemon chia sparkling water.