Preheat the oven to 190 Celsius (370 Fahrenheit).
Remove the skin from the butternut squash using a good vegetable peeler. Remove the seeds and chop the squash into bite sized cubes. Place in a baking tray with some olive oil and roast for 40 minutes, turning halfway.
Place the chicken breasts in a baking tray, using the olive oil to coat the try and a little on top of the chicken. Crush the garlic over the chicken and add some salt and pepper. Place in the oven next to the butternut squash for 25-35 minutes, turning halfway.
While the butternut and chicken are cooking, chop the lettuce into slices and wash them. Leave them to dry while you prepare the rest of the salad.
Prepare a plate for each serving. Wash the cherry tomatoes and cut in half. Place 6 tomatoes on each plate.
Cut the avocado into quarters, removing the seed. Remove the skin from each quarter, then cut into thinner slices. Place a quarter onto each plate.
Remove the skin from the mango. Slice it into small pieces, avoiding the seed in the centre. Place a quarter onto each plate.
Once the lettuce is dry, place a quarter onto each plate.
Remove the butternut squash from the oven and place a quarter on each plate.
Remove the chicken from the oven and cut through the breasts to ensure they're fully cooked and safe to eat. Slice the breasts into thin slices and place half of each breast onto each plate. Serve with mayonnaise.