Preheat a grill or griddle pan over medium heat.

In a small bowl, whisk together the garlic powder, dried oregano, lemon juice, and olive oil to make the marinade. Season with salt and black pepper to taste. Adjust seasonings so the marinade tastes flavorful on its own.

Set aside half of the marinade to brush on the buns later.

Brush one side of the chicken breast with the marinade and season with salt and pepper.

Place the brushed side of the chicken onto the hot grill or griddle pan. Brush the top side with additional marinade. Cook for about 6 minutes, or until grill marks appear.

Flip the chicken, brush with more marinade, and continue cooking for another 6 minutes, or until the chicken reaches an internal temperature of 165°F (74°C). Once cooked, remove from heat and 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 sandwich: Spread mayonnaise on each bun. Layer with butter lettuce, tomato slices, grilled chicken, Swiss cheese, and red onion.

To make the homemade lemonade, combine fresh lemon juice, water, and sugar in a pitcher. Stir until the sugar is dissolved. Add ice cubes if desired.

Serve the grilled Greek chicken sandwiches with carrot sticks, hummus, pickle spears, and a refreshing glass of lemonade.