Turn the block of halloumi on its side and cut it into three to four even slices. Set aside.
Pour the olive oil, salt, and pepper into a large plastic bag. Put the tomatoes and chicken in and massage until everything is evenly coated in the seasoned oil.
Dump everything onto a foil lined baking sheet. Top the chicken with the large basil leaves and then the slices of halloumi.
Bake for 15 minutes at 350˚, then turn on the broil setting and cook for another 10 minutes or until the cheese is golden brown and the chicken is cooked. (note: oven times vary, especially broil so keep an eye on it!)
Take the chicken out and let it rest on a plate and continue to cook the tomatoes for 10 more minutes back on 350˚ F. (They will become super sweet and have a great concentrated tomato flavor!)
Meanwhile, add the olive oil, garlic, salt, pepper, and lemon juice to a blender and blitz for 30 seconds. Next add the basil and blitz until it's the consistency of pesto, about 30 seconds.