Preheat the oven to 200°C/gas mark 6. Toss the pumpkin in ½ tbsp of oil, then spread out on a baking tray. Roast for 30 minutes until tender and golden.
Meanwhile, heat 1 tbsp of oil in a medium-sized pan over a medium-high heat. Add the leeks and cook for 6 minutes until softened and just starting to turn golden. Add the farro and stir to coat in the oil. Add the sherry and let it bubble, stirring, until it has almost evaporated. Add the stock, stir, lower the heat and then cover, leaving the farro to simmer for 20 minutes.
Put the kale, sage, garlic and lemon juice in a mini processor and blitz until finely chopped. Continue to pulse, gradually pouring in the oil until you have a loose, vibrant pesto. Season, then cover and store in the fridge until needed.
Put ¼ of the roasted pumpkin in a food processor and blitz until smooth. Remove the risotto from the heat and gently stir in the pumpkin puree. Gently fold in the remaining roast pumpkin and season. Divide between four serving bowls and top each with a spoonful of the pesto.