How to Make It
Fill a large bowl with water. Squeeze in the juice from the lemon halves, then add the lemon halves to the water. Working with 1 artichoke at a time, peel off the dark green outer leaves. Cut off the top fourth of the artichoke; peel and trim the stem. Slice the artichoke lengthwise 1/8 inch thick and drop into the lemon water. Repeat with the remaining artichokes.
In a saucepan, heat the oil. Add the onion, garlic and lardo and cook over moderate heat, stirring, until softened, about 10 minutes. Drain the artichokes; discard the lemon. Add the artichokes to the saucepan, cover and cook over moderately low heat, stirring, until almost tender, about 10 minutes. Add the white wine, cover and simmer over moderately low heat until tender, about 5 minutes. Season with salt and pepper.
Meanwhile, in a large pot of boiling salted water, cook the spaghetti until al dente. Drain the pasta, reserving 1/4 cup of the pasta cooking water. Add the pasta and the reserved cooking water to the artichokes and toss over moderate heat for 1 minute. Remove from the heat, stir in the 1/4 cup of cheese and season with salt and pepper. Serve the pasta, passing more grated cheese at the table.