Trim the chicken breast of any visable fat and remove the fillet if the butcher did not already do so.
Place the chicken between two pieces of plastic wrap, or into a small ziplock bag. Using a rolling pin or meat mallet, gently pound the thicker end of the chicken so the chicken breast is an even thickness.
Generously season both sides of the chicken breasts with salt and pepper, then place into a small baking dish. The chicken should be crowded, but not overlapping. I use a 7x9 dish, but 8x8 will work well too.
Pour the olive oil and lemon juice into a small bowl or measuring cup, then finely mince the shallot and garlic.
Remove the leaves from the stems of the thyme and add the thyme, shallot and garlic to the olive oil and lemon juice. Stir well.
Pour the marinade over the chicken. Turn the chicken a few times to make sure it is fully coated then cover the dish and refrigerate a minimum of one hour, or overnight.
Heat the grill to medium high heat. Remove the chicken from the marinade and place on the grill.
Cook 8-10 minutes then flip and cook an additional 8-10 minutes until internal temperature reads 165°F.
Remove the chicken to a plate, cover with foil and let rest for 5 minutes before serving.