Bring a pot of lightly salted water to a boil. Add pasta and cook 9 to 12 minutes, or until just tender; rinse and drain. Return pasta to pot and toss with 1 tablespoon olive oil.
Meanwhile, in a large skillet, heat remaining 1 tablespoon olive oil over medium-high heat. Add onion and cook 3 to 5 minutes, or until onion softens and is lightly browned. Stir in tomatoes, beans, and garlic salt. Bring to a boil; reduce heat to low and simmer 10 minutes. Add spinach and cook 2 minutes, or just until it wilts.
Toss tomato-bean mixture with pasta. Transfer to serving bowl and garnish with mozzarella.