Preheat the oven to 200°C/fan 180°C/gas mark 6. Season the chicken breasts well and leave at room temperature for 10 minutes. Brush a griddle pan with 1 tsp oil and place over a medium-high heat. Once hot, add the chicken breasts and cook for 4 minutes on each side until well seared, then transfer the entire pan to the oven. Cook for 10 minutes or until completely cooked through. Remove from the oven and leave to rest for 10 minutes.
Meanwhile, heat a small frying pan over a medium heat. Add the fennel seeds and toast for 2 minutes until fragrant. Instantly transfer to a pestle and mortar and roughly grind.
Whisk together the toasted fennel seeds, lemon juice, vinegar, honey and mustard in a medium bowl until combined. Drizzle in the remaining oil, whisking continuously until emulsified.
Begin to segment the grapefruit by cutting off a little of the top and bottom to give two flat edges. Stand it on one of the flat surfaces and cut away all the skin and pith. Cut along the curve of the fruit, being careful not to remove any flesh. Use the knife to gently slice one side of a segment. Cut until you reach the middle of the fruit, ensuring you do not pierce the membrane. Repeat on the other side of the segment until loose and remove. Repeat with the remaining segments.
Peel the avocados and slice in half lengthways. Place an avocado half cut-side down on a chopping board and thinly slice lengthways. Fan out the slices so you have a long length of avocado and curl the slices in from one end towards the centre to create a rose shape. Repeat with the remaining halves.
Slice the chicken into strips. Add the avocado rose to the plate and place the spinach, chicken and grapefruit segments around it. Drizzle with the dressing to serve.