Chop one-third of onion very finely, and thinly slice remaining onion.
In medium bowl, combine chopped onion, egg, bread crumbs, dried tomatoes, 1 teaspoon Italian seasoning and ground beef. Mix well.
Shape into 12 meatballs.
In large skillet, add oil and heat over medium. Brown meatballs. Drain and discard fat.
Transfer meatballs to 4-1/2 to 5-1/2 quart slow cooker. Add sliced onion.
Cut off and discard upper stalks of fennel. Save some of the feathery leaves for garnish (optional).
Remove wilted outer layers, remove thin slice from fennel base. Cut fennel into thin wedges.
Stir fennel, chicken broth, white pepper, garlic, and remaining Italian seasoning into slow cooker.
Cover and cook on low heat setting 8 to 10 hours or on high heat setting 4 to 5 hours.
If using low-heat setting, turn to high-heat setting. Gently stir in orzo. Cover and cook about 15 more minutes or until orzo is tender. Stir in spinach. Transfer to serving bowls and garnish with reserved fennel leaves, if desired.