In a deep skillet or dutch oven mix butter and flour together over low heat to form a roux ( a thick, smooth, bubbly mixture). Add the onions, celery, and garlic and sauté until tender. Add the tomatoes, salt, pepper, cayenne, and water. Simmer covered for 20 - 30 minutes. Add fillets and cook until tender and flaky, 15 -20 minutes. Serve on a bed of rice.