Finely chop one-third of the onion; thinly slice the remaining onion. In a large bowl, combine chopped onion, egg, dried tomato, bread crumbs, and 1 teaspoon of the Italian seasoning. Add ground beef; mix well. Shape into 12 meatballs. In a large skillet, cook meatballs in hot oil over medium-high heat until brown on all sides. Carefully drain off fat. Transfer meatballs to a 4 1/2- or 5-quart slow cooker.
Meanwhile, cut off and discard upper stalks of fennel. If desired, reserve some of the feathery leaves for garnish. Remove any wilted outer layers; cut off a thin slice from fennel base. Cut fennel into thin wedges; add to cooker. Add sliced onion, the remaining 1 teaspoon Italian seasoning, broth, the water, garlic, and pepper.
Cover and cook on low-heat setting for 8 to 10 hours or on high-heat setting for 4 to 5 hours.
If using low-heat setting, turn to high-heat setting. Gently stir in uncooked pasta. Cover and cook for 20 to 30 minutes more or until pasta is tender.
Stir in spinach. If desired, garnish each serving with the reserved fennel leaves.