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 3 tablespoons EVOO, 3 turns of the pan, over medium heat. Add the yellow onion and garlic and cook for 2 minutes. Stir in the sliced fennel and the sardines and cook until the fennel is softened, about 7 minutes. Stir in the tomatoes, chicken broth, wine, raisins and salt and pepper to taste. Simmer for 5 minutes. Stir in the butter.
In a salad bowl, whisk together the mustard and vinegar and the remaining 3 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.