In a Dutch oven heat oil over medium-low heat. Add onion. Cook, covered, for 13 to 15 minutes or until onion is tender, stirring occasionally. Cook, uncovered, over medium-high heat for 3 to 5 minutes or until onion is golden, stirring frequently. Add garlic and salt; cook and stir for 30 seconds.
Stir in beans, broth, and tomato paste; cook for 2 minutes. Stir in mustard greens; cook for 2 minutes. Stir in spinach and tomato; cook about 1 minute more or until spinach begins to wilt.