Cook pasta in plenty of salted water until al dente. Drain in a colander.
Meanwhile, heat a frying pan over medium-high heat, add oil and onions and cook 1 minute until onions soften. Add pinenuts and continue to cook until they become golden, then add raisins and cook until they begin to swell and caramelise, stirring frequently.
Peel rind off one lemon with a potato peeler and finely julienne it, then juice both lemons. Add lemon juice and zest to onion mixture and cook over medium heat 3–4 minutes, stirring well. Add butter and mix in well.
Tip cooked and drained pasta into a large bowl and mix in spinach and salt, then the ‘fsauce’. Spoon into four bowls and add freshly grated parmesan if desired.