Melt the butter in a large skillet with a tight-fitting lid over medium heat; add onion and garlic and sauté for 3 to 4 minutes or until soft.
Add oregano, cumin and crushed red pepper flakes; sauté for 1 to 2 minutes. Stir in broth, tomatoes and tomato liquid; bring to a boil then reduce heat and simmer for 5 minutes. Add corn; stir.
Arrange fish fillets over broth mixture and spoon some liquid over the top; cover and simmer for 5 minutes or until fish flakes easily when tested with a fork.
To serve: Set each fillet in a large soup bowl; spoon broth around it; sprinkle with chopped cilantro and garnish with tortilla strips.