As a note, I like to use thin-sliced chicken breasts for this, because it’s what I prefer to bite into. You can use 4 large chicken breasts or 4 thin chicken breasts - use what you like! If you only have 2 large chicken breasts, you can also slice them lengthwise to make 4 pieces.
Place the chicken in a dish or resealable bag to marinate. Whisk together the olive oil, dijon, garlic and a big pinch of salt and pepper. Pour the marinade over the chicken and refrigerate for at least 30 minutes or even overnight. Remove the chicken from the fridge about 30 minutes before grilling. At this time, I like to prep the caesar dressing. You can make it ahead of time and store it in the fridge too.
Preheat the grill to high and let it heat for 10 to 15 minutes.
Remove the chicken from the marinade, letting the excess drip back into the dish. Place the chicken on the grill and grill about 3 to 5 minutes per side (depending on thickness), until the internal temperature reaches 165 degrees F. Remove the chicken and let it rest for 5 to 10 minutes. I also like to grill the brioche buns too!
To assemble, I like to add some caesar dressing on the bottom of the bun. Top with romaine, then top with the chicken. Spritz with lemon if you wish. Drizzle with caesar dressing and sprinkle on parmesan cheese. Top with the bun and serve!