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Heat half the oil in a shallow saucepan over a low-medium heat and scatter in the sage leaves, if using. Fry for 2 mins until crisp, lift onto a plate using a slotted spoon. Tip the onion into the pan and cook for 3 mins until soft, then add the garlic and cook for 1 min more. Sprinkle in the pepper and toast for 1 min, then stir in the beans. Turn off the heat. Stir in the lemon juice.
Season the mixture with salt, then tip into a food processor or mini chopper with the remaining oil. Blitz everything together to your desired consistency – it can be a smooth or a rough paste. Will keep covered in the fridge for two days. Bring to room temperature 1 hour before serving.
Serve the dip as is with crostini or breadsticks, or drizzle with a little extra olive oil and top with the crispy sage leaves, if using.