Heat a large skillet over medium-high heat. Add oil to pan; swirl to coat. Add sliced onion, and sauté 5 minutes or until lightly browned, stirring occasionally. Add garlic to pan; sauté 30 seconds. Stir in beans and 1/4 teaspoon salt; cook 1 minute. Add spinach to pan; cook 1 minute or until spinach wilts and beans are thoroughly heated.

Combine tomato, chopped onion, cilantro, lime juice, 1/8 teaspoon salt, and jalapeño in a small bowl. Warm tortillas according to package directions. Arrange 1/3 cup bean mixture in center of each tortilla. Top each taco with 3 tablespoons tomato mixture and about 1 tablespoon cheese.