Preheat the oven to 200C (400F).
Cut the chicken breasts into smaller pieces if required. Season with salt and pepper (if using). Add a tablespoon of olive oil to your skillet and fry the chicken each side until light golden brown (you may need to do this in two batches if your skillet is on the smaller size). About 10-12 minutes overall depending upon the size of the fillets. Set aside on a plate.
Reduce the heat to low and, in the same skillet, fry the onion and fennel in olive oil until softened, about 5 minutes.
Add the garlic, blueberries, maple syrup, apple cider vinegar, salt and pepper. Bring to a light simmer, and cook for around 10 minutes until the blueberries have broken down into around half liquid.
Add your chicken to an ovenproof dish and then spoon the blueberry sauce over it. Bake in the oven for 20 minutes, or until the chicken has an internal temperature of 165F.