Start by whisking together your marinade ingredients in a bowl. Combine ketchup, mayonnaise, soy sauce, buttermilk, salt, and pepper. This mixture will infuse the tilapia with incredible flavor, so take your time to blend well. Next, place the tilapia fillets inside a large ziplock bag or bowl, pour in the marinade, seal it up, and let it marinate in the fridge for an hour and a half. The longer it marinades, the more flavor it absorbs.
While the fish is marinating, it’s time to prepare your vegetables. Begin by slicing the onion into thin half circles. Then, slice the bell peppers into thin strips and julienne or grate the carrots. Having everything prepped and ready makes the cooking process smoother and more efficient.
Heat a large skillet over medium-high heat and add about 2 to 3 tablespoons of olive oil. Once the oil is hot, add the sliced onion to the pan. Sauté for about 3 minutes until the onions are softened and translucent. Don’t rush this step; letting the onions caramelize a bit enhances their flavor.
Next, toss in the sliced bell peppers and carrots to the skillet. Continue to sauté for another 4 to 5 minutes, allowing the vegetables to soften. They'll release their natural sweetness as they cook, adding depth to the dish.
Once the vegetables are nicely cooked, remove them from the skillet and set aside. This step ensures they don’t overcook in the oven.
Now, it’s time to cook the marinated tilapia. You might want to drain and discard the marinade. In the same skillet, add 2 to 3 tablespoons of olive oil and heat it over medium-high. Gently add the tilapia fillets to the skillet, cooking them for about 2 minutes on each side until they are golden brown. It’s okay if the fish is not fully cooked through at this point.
Next, prepare your casserole dish. Start by layering half of the sautéed vegetables on the bottom. Lay the cooked tilapia on top of the vegetables, ensuring even coverage.
In a small bowl, mix 1 tablespoon of ketchup with 1 tablespoon of mayonnaise. Brush this mixture evenly over the top of the tilapia fillets to create a flavorful topping.
Cover the fish with the remaining sautéed vegetables, making sure to distribute them evenly. Sprinkle the top with additional salt and pepper to taste.
Finally, cover the casserole dish tightly with foil or a lid and place it in a preheated oven at 375 degrees Fahrenheit. Bake for about 15 minutes, or until everything is heated through and the tilapia flakes easily with a fork. This will result in a perfectly cooked casserole that retains all its moisture and flavors.
