Heat your oven to 200°C/Fan 180°C/Gas 6. Set a roasting tray on the top shelf to heat up.
Peel your sweet potatoes. Cut into 3cm cubes. Tumble onto the warmed roasting tray. Gloss with 1 tbsp oil. Season well. Roast for 30 mins or till tender and golden round the edges.
Once the sweet potatoes are done, sprinkle over 2 tsp Cajun spice (add more or less, to suit your taste). Return to the oven for 2 mins, then take out of the oven and set aside to cool.
Grate the zest of your lemon into a large bowl. Squeeze in 1 tbsp juice. Whisk with 1 tbsp oil and a pinch of salt. Set aside.
Halve and de-stone your avocado. Scoop the flesh into a blender or food processor. Add a pinch of salt and a good squeeze of lemon juice. Blend till smooth. Taste. Add more lemon juice or salt, as needed.
Dollop the avocado dressing onto a large platter or 2 plates. Toss the watercress and sweet potato into the large bowl with the lemon dressing. Gently mix through the dressing. Pile on top of the avocado dressing. Serve.