Put the olive oil in a saucepan over medium heat.

When hot, stir in the chopped onion and season with a pinch of salt and a grind of pepper.

Cook, stirring often, until the onion is starting to soften and colour (approx. 10 min): add the garlic for the final 2 - 3 min. Tip: for greater depth of onion flavour you can cook longer at this stage if liked.

Stir in the drained cannellini beans, 2 tsp of the chopped rosemary, plus another grind of pepper and cook for another minute. Optional: add a little chopped lemon zest.

Stir in stock to only just cover the beans, turn up the heat and bring to a boil. Turn down to a simmer and cook (without a lid) until the onion is completely soft and the stock has reduced to just a few tablespoons coating the beans (10 - 15 min). Tip: if there's too much liquid at the end, turn the heat up and bubble vigorously for a minute or two.

Using a slotted spoon (or pass through a fine sieve over a jug), transfer the beans, onions etc. to a blender and squeeze in the juice of half the lemon. Whizz to a puree, adding as much of the cooking liquor as needed to get a smooth but not runny consistency.

Taste and adjust to your liking with more salt, pepper, rosemary or lemon.

Best served warm, the puree can be made in advance and kept in the fridge for 2 - 3 days. Reheat gently in a saucepan or microwave. Freezing not recommended.

Optional: sizzle a little more chopped rosemary in olive oil or butter and drizzle over the puree just before serving.