Add 3 tablespoons oil to a heavy skillet on medium-high heat. Add onions, bell peppers, and celery. Saute vegetables until wilted, about 6 to 8 minutes. Remove vegetables from skillet and set aside.
Return skillet to stovetop. Make a roux with the remaining oil and flour. Cook, stirring constantly, until the roux is a dark brown, about 15 minutes. Add the wilted vegetables and cook about 5 minutes longer.
Add the tomatoes and juice. Zest and juice lemon and add to the mixture. Add garlic and bay leaves. Simmer for 45 minutes until the courtbouillon thickens.
Add the green onions and cook an additional 5 minutes. Add salt and pepper to taste.
Season the redfish fillets with salt and pepper. Place the redfish fillets into the courtbouillon. Cover and cook for about 5 to 10 minutes until the fish turns opaque.
Serve over hot rice.