Preheat the oven to 200°C (fan-assisted)/42°F/gas 7.
Trim the top from the leeks at the point they start to flare (don’t throw these away, you can use them in lots of other recipes!). Place on a baking tray and roast for 35-40 minutes, or until fully charred on the outside.
Heat 2 tbsp of olive oil in an oven-proof pan over a medium heat. Once hot, add the flour and garlic and whisk vigorously to form a paste. Continue whisking for a minute or so, to cook off the flour.
Next, add the oat milk a little at a time, whisking continuously, until smooth. Add the white wine and rosemary sprig, then bring the sauce to a gentle simmer and cook, stirring frequently, for 5-6 minutes, or until thickened and glossy. Remove the rosemary from the sauce, then stir in the nutritional yeast. Season to taste with salt and black pepper.
Chop each burnt leek in half lengthways and use a spoon to scoop out the soft flesh in the middle. Roughly chop the leek flesh before adding it to the sauce along with the cannellini beans. Stir well to combine, then transfer the whole pan into the oven and bake for 15-20 minutes, until golden on top. Serve with crusty bread and a good crack of black pepper.
