Place 1/2 cup spinach into each of 6 bowls; set aside.
Steam potatoes, covered, 10 minutes or until tender. Place 8 potato quarters into each bowl.
Combine tomatoes, broth, wine, onions, and garlic in a large nonstick skillet over medium-high heat. Bring to a boil, and cook until reduced to 3 cups (about 25 minutes). Add salt, pepper, and saffron to pan; cook 2 minutes. Reduce heat to medium; arrange fish pieces in a single layer in pan. Cover and cook 8 minutes or until fish flakes easily when tested with a fork or until desired degree of doneness. Divide fish evenly among bowls; spoon 1/2 cup broth mixture over each serving.