Heat a large nonstick skillet over medium-high heat; add first 4 ingredients. Bring mixture to a boil, and cook 8 minutes, stirring constantly. Reduce heat to medium-low; stir in milk, and cook 1 minute. Remove from heat, and stir in 1 1/2 teaspoons butter. Set aside, and keep warm.
Rinse and dry skillet. Heat skillet over medium-high heat; add cumin, salt, pepper, and 1 tablespoon butter, stirring well. Add fish, and cook 5 minutes on each side or until fish flakes easily when tested with a fork.
Place fish on individual serving plates; spoon sauce evenly over fish. Garnish with orange slices, if desired. Serve immediately.