Part 1: Preparing the Components

1. Prepare the Topping First: In a small bowl, combine the crushed Ritz crackers with the 4 tablespoons of melted butter. Use a fork to toss them together until all the crumbs are evenly moistened. Set this bowl aside. This ensures your topping is ready to go when you need it.

2. Cook the Vegetables: Cook the two bags of frozen vegetables according to the package directions, either by boiling or steaming in the microwave. The goal is to cook them until they are tender-crisp, not soft or mushy. They will continue to cook in the oven, so slightly undercooking them is better than overcooking.

3. Drain the Vegetables Very Well (Crucial Step): Once the vegetables are tender-crisp, drain them thoroughly in a colander. Gently press on them with the back of a spoon to remove any excess water. Let them sit in the colander to steam dry for a few minutes. This is the most important step to prevent a watery casserole.

4. Preheat and Prepare the Dish: Preheat your oven to 375°F (190°C). Generously grease a 9×13 inch baking dish with butter or non-stick spray.

Part 2: Creating the Velvety Velveeta Sauce

1. Sauté the Onion: In a medium saucepan or Dutch oven, melt the 1 tablespoon of butter over medium heat. Add the finely chopped yellow onion and cook, stirring occasionally, for 5-7 minutes, or until the onion is softened and translucent.

2. Build the Sauce: To the saucepan with the cooked onions, add the cubed Velveeta cheese, the can of condensed soup, and the milk.

3. Melt to Smooth Perfection: Continue to cook over medium-low heat, stirring frequently with a whisk or rubber spatula, until the Velveeta is completely melted and the sauce is smooth, creamy, and velvety. This will take about 5-7 minutes. Be patient and keep the heat low to prevent the sauce from scorching on the bottom. Once the sauce is smooth, stir in the salt, black pepper, and optional garlic powder.

Part 3: Assembling and Baking the Casserole

1. Combine and Pour: Place the well-drained, cooked vegetables into the saucepan with the cheese sauce. Gently stir everything together until the vegetables are evenly coated in the glorious, creamy sauce.

2. Fill the Baking Dish: Pour the vegetable and cheese mixture into your prepared 9×13 inch baking dish and spread it into an even layer.

3. Add the Topping: Sprinkle the prepared buttery cracker topping evenly over the entire surface of the casserole, from edge to edge.

4. Bake to Bubbly Gold: Place the casserole in the preheated oven and bake, uncovered, for 25-30 minutes. The casserole is done when the sauce is hot and bubbly around the edges and the cracker topping is a beautiful, rich golden brown.

5. Let It Rest: Remove the casserole from the oven and let it stand for 5-10 minutes before serving. This allows the molten cheese sauce to set up slightly, making it easier to serve.