Scrub mussels thoroughly with a scrub brush, removing beards. Discard any opened shells.
Sauté garlic in hot oil in a Dutch oven over medium heat 1 to 2 minutes. Add tomatoes, chopped basil, and next 2 ingredients; simmer, stirring occasionally, 5 minutes.
Add mussels and spinach; cook, covered, 5 minutes or until all mussels have opened. Remove from heat. Serve with Italian bread. Garnish, if desired.