Combine the first 4 ingredients in a small bowl, stir well and set aside.
Pat fillets dry with paper towels. Using 1 tablespoon mustard, spread a thin layer of mustard over fillets; set aside. Combine broth, remaining 3 tablespoons mustard and garlic in a large skillet; bring to a boil. Add rice, cover, reduce heat and simmer 10 minutes. Stir in spinach. Arrange fillets on top of rice mixture; cover and simmer about 8 minutes or until liquid is absorbed and fish flakes easily with a fork. Spoon tomato mixture over top of fish before serving.
