Cut about ¾ quarter of the way through each chicken breast. Be careful not to cut all the way through.
Season chicken breast with salt and pepper and fill each with cheese and tomato.
Seal with 2 or 3 toothpicks to secure the filling inside while cooking.
Heat 2 tablespoons of olive oil in a large non-stick skillet over medium-high heat.
Add the chicken and fry each side for about 2 minutes until golden brown.
Mix garlic, balsamic vinegar and brown sugar together in a small bowl. Pour the mixture in the skillet around the chicken. Bring to simmer and cook chicken each side for about 1 minute while stirring occasionally until the glaze has slightly thickened. It will not take longer than 3 minutes.
If you are using a heat proof pan, transfer it in the oven directly to cook the chicken for a further 10 minutes. If not, transfer chicken to a baking dish before transferring in the oven.
Use a sharp knife to cut chicken breast through by starting on the thickest side and work your way in.
Do not overcook or it will become dry.
Use chicken breast of even size and even thickness so they cook at the same rate.
You can shred the cheddar if you find it easier to work with.