Start by peeling and chopping the onions and garlic cloves. Trim and finely slice the leek and give it a thorough rinse in a colander to get rid of any dirt. Trim and slice the spring onions into 1cm pieces. Place the olive oil in a large saucepan over a medium-low heat. Add the onion and garlic frying for a few minutes to soften before adding the leek and spring onions. Season with salt and sauté for 15 minutes, until softened and sticky but not coloured. When the vegetables are ready, stir in the frozen spinach, and leave to defrost for 5 minutes with the lid on. Stir in the tomato purée, sugar and smoked paprika. Fry for a few minutes, then stir in the washed rice and cook for a further minute. Add the passata and 750ml of boiling water, then crumble in the stock cubes and season with salt and pepper to taste. Turn the heat up and bring to the boil. Cook for 30 minutes, stirring once halfway through and adding more water if it looks like the rice has dried out and is sticking to the bottom of the pan. After half an hour, the rice should be cooked, give it a stir and check the texture – it should feel like a paella. If it is a little on the wet side, cook with the lid off to reduce it. Remember the rice will absorb liquid as it cools, so don't reduce the liquid too much. Squeeze in the lemon juice, taste and tweak the seasoning, then leave for 5 minutes before serving. Finely chop the dill and stir most of it through the rice, leaving a little to sprinkle on each serving as a garnish. Serve warm.