Preheat the oven to 220C/425F/Gas 7.
Heat a griddle pan until very hot.
Season the chicken breast with salt and freshly ground black pepper. Brush the breast with a little of the olive oil and then place the chicken breast into the griddle pan. Cook for 8-10 minutes, or until completely cooked through.
Meanwhile, place the avocado into a small ovenproof frying pan and place into the oven to bake for 5-6 minutes, or until heated through. Remove from the oven and, when cool enough to handle, carefully peel off the skin.
For the mustard dressing, place the olive oil, sugar, mustard and vinegar into a screw-topped jar. Season generously with salt and freshly ground black pepper, screw on the lid well and shake until the dressing is emulsified.
Cut the cherry tomatoes in half and arrange on a plate. Slice the baked avocado and arrange over the top of top of the tomatoes, then add the seared chicken. Spoon over the mustard dressing and serve.