Add garlic to a drizzle of oil in a large oven-safe pan over medium heat. Fry for a couple of mins, then add chopped tomatoes, oregano, sugar, salt, pepper and chilli flakes (optional). Simmer over low-medium for 20-25mins, or until very thick. Pre-heat oven to 200C/390F
Meanwhile, divide the mozzarella in two. Slice or grate one half to top the chicken, then slice the other half into 20 thin wedges around the same width as the chicken. Slice the chorizo into 20 thin wedges.
Make 5 slices into each breast with an even distance apart, slicing around 3/4 of the way down. Season all over with salt and pepper. Wedge a basil leaf into each gap, then add the chorizo and mozzarella.
Once the sauce is thick (should just about cover the base of the pan) add the chicken on top. Drizzle with oil then place in the oven for 15mins. Remove, top with cheese and continue baking for another 10mins, or until the cheese starts to bubble and the chicken is piping hot and white through the centre. If making the side dish combine everything in a bowl at this point.
Remove chicken and rest to one side, then quickly stir the sauce to emulsify the chorizo fat with the tomato sauce (can rapidly simmer to thicken more if needed).
Serve chicken on top of sauce then tuck in and enjoy.