How to Make It
In a large, deep skillet, melt the butter in the olive oil over low heat. Add the onions and cook, stirring occasionally, until softened and golden, about 20 minutes. Add the sardines and water and cook, stirring, until hot, about 2 minutes. Add the milk and simmer until the sardines dissolve into a thick sauce, about 6 minutes. Season with salt and pepper.
Meanwhile, in a large pot of boiling salted water, cook the spaghetti until al dente. Drain and add the spaghetti to the skillet. Toss the spaghetti with the sauce and season with salt. Transfer to bowls and serve at once with freshly ground pepper.