Preheat oven to 350 degrees.: As the oven warms, you will notice a faint heat ripple and a dry warmth that helps the casserole develop an even bake. Preheating ensures the dish goes into a consistent environment so the cheese melts evenly and the topping crisps. If you skip preheating you risk uneven cooking, where the filling might be lukewarm while the top browns too quickly. Check the oven gauge early, because an oven running cool will lengthen bake time and alter texture.
Cook lima beans according to package directions.: When you cook the lima beans , listen for a gentle simmer and watch for the beans to become tender but still hold shape. Properly cooked beans will have a soft, buttery yield when pressed between a spoon and the roof of your mouth. Overcooking can make them mushy and cause the casserole to lose structure, so time them carefully and drain well to avoid adding excess water to the mixture. If beans are undercooked they will be starchy and detract from the silky final texture.
Cook onion in butter until soft.: As the diced onion sizzles in the butter , you will smell a sweet aroma and see the pieces turn translucent. This step develops sweetness and mellows harsh onion bite, which deepens the casserole flavor. Stop once the onion is soft and glossy, not browned, to keep a gentle savory base. A common mistake is to rush this step, leaving raw onion sharpness in the casserole, so take your time and stir to prevent sticking.
In a large bowl, combine lima beans, soup, milk, onion, Worcestershire sauce, pimentos, black pepper and 1 cup cheddar cheese.: When you stir these components, note how the condensed cream of celery soup and milk fuse into a smooth, pale sauce that coats each lima bean . The aroma will deepen with the savory hint from the Worcestershire sauce and the sweetness from the pimentos . Combining off heat helps preserve the cheese texture until baking. A potential pitfall is overworking the beans, which can break them down, so fold gently until everything is evenly distributed.
Transfer to a 1 1/2 to 2-quart lightly greased casserole dish.: As you spoon the filling into the dish, you'll feel its creamy weight and see it settle into an even layer. Greasing the dish prevents sticking and promotes an easy release when serving. Spread the mixture smoothly so the top is level, which helps the cheese and crackers brown uniformly. If the dish is too full, it may bubble over, so use the recommended size to avoid a messy oven.
Sprinkle remaining cheese and the crackers on top. Cover with foil and bake 20 minutes. Uncover and place bake in oven for 10 minutes.: The moment you add the remaining cheddar cheese and the coarsely crushed Ritz crackers , the casserole visually shifts from purely creamy to promising a crisp crown. The crackers should be distributed so there are gaps for hot air to crisp them during baking. Pressing them lightly helps them adhere, but avoid packing them down, which can prevent proper browning. Overly thick topping layers may toast unevenly, so aim for a thin, even scatter.
Cover with foil and bake 20 minutes: Covered baking steams the interior, allowing the filling to heat through and meld without drying out, and you should hear a soft bubbling around the edges as it begins to set. The foil traps moisture so the beans remain tender and the cheese melts into silky pockets. A common error is to leave the dish covered for too long, which can prevent the topping from crisping later, so stick to the time and monitor the bubbling as an indicator that the center is hot.
Uncover and place bake in oven for 10 minutes: Once uncovered, the oven's dry heat will transform the topping into a golden, crunchy layer and bring the cheese to a bubbly finish. Watch for a fragrant toasty smell and a golden hue on the crackers as signs of readiness. Remove the dish when the crackers are crisp and the cheese has small, molten bubbles; leaving it too long can dry the filling. A common problem is misreading the visual cues, so rely on color and bubbling rather than time alone to judge doneness.