While your grill or grill pan heats up, trim any excess fat from the chicken breasts and cut them in half lengthwise to create thinner, more evenly-cooked pieces. In a bowl, whisk together the olive oil, lemon juice, italian seasoning, garlic powder, smoked paprika, salt, pepper, and honey to create a balanced marinade with both acidity and sweetness. Place the prepared chicken pieces in the marinade and let them sit for at least 20-30 minutes at room temperature—this allows the flavors to penetrate the meat while it comes closer to room temperature for more even cooking.

While the chicken marinates, prepare the spread by combining mayonnaise, barbecue sauce, and honey mustard in a small bowl with a pinch of salt and black pepper. Whisk until smooth and well combined. I like to taste the sauce at this point and adjust the honey mustard or barbecue sauce ratio if needed—this sauce is what ties everything together, so getting it right makes a huge difference in the final sandwich.

Heat a grill or grill pan to medium-high heat (around 375-400°F if using a grill pan). Remove the chicken from the marinade, allowing excess liquid to drip off, and place the pieces directly on the hot grill. Cook for 3-4 minutes per side until the internal temperature reaches 165°F and you have nice golden-brown grill marks. The chicken will continue to cook slightly after removal, so don't overcook it. Transfer the cooked chicken to a plate and let it rest for a few minutes—this keeps the chicken juicy.

While the chicken rests, melt the butter in a skillet or griddle over medium heat. Split the brioche buns in half and place them cut-side down on the butter, toasting until golden brown, about 1-2 minutes. Don't walk away—brioche browns quickly and you want them toasted but not burnt. Transfer to a clean plate.

Spread the sauce from Step 2 on the toasted bun bottoms. Layer each sandwich with a leaf of lettuce, a piece of grilled chicken from Step 3, a slice of tomato, and a few rings of red onion. Place the bun tops on and serve immediately while the chicken is still warm. The warm chicken will slightly soften the fresh vegetables—this contrast of temperatures and textures is what makes the sandwich special.