Bring a large pot of water to a boil, salt it, add the spaghetti and cook until al dente. Drain.
While the pasta is working, in a large, deep skillet, heat three tablespoons EVOO, about three turns of the pan, over medium heat. Add the yellow onion and garlic and cook for two minutes. Stir in the sliced fennel and the sardines and cook until the fennel is softened, about seven minutes. Stir in the tomatoes, chicken broth, wine, raisins and salt and pepper to taste. Simmer for five minutes. Stir in the butter.
In a salad bowl, whisk together the mustard and vinegar and the remaining three tablespoons EVOO. Add the spinach and red onion; toss.
Toss the spaghetti with the sardine-fennel sauce; top with the pine nuts and fennel fronds. Serve the salad alongside.