Heat your oven to 220°C/Fan 200°C/Gas 7. Set a large roasting dish on the top shelf to heat up.
Peel the beetroot. Cut into 2-3cm pieces. Tumble into the warmed roasting dish with 1 tbsp oil. Season well. Pop in the oven to roast for 20 mins.
Halve, peel and finely chop the onion. Add it to the beetroot. Roast for a further 10 mins, or till the beetroot is tender and caramelised around the edges. Peel and finely chop or grate the garlic. Swirl it through the warm beetroot – it'll cook in the residual heat.
Tumble the beetroot mix into a blender or food processor with a 2cm chunk of peeled ginger, the zest of the lemon and 500ml stock (homemade or using the stock cube). Blend till smooth, trickling in more stock or warm water, till it's as thin or as thick as you like it.
Make up a zingy dressing to finish your soup: simply whisk 2 tbsp lemon juice with 2 tbsp oil, a pinch of salt and 1 tsp freshly grated ginger.
Dish up the soup and drizzle a swirl of dressing over the top.