To achieve a perfectly thick white sauce, try using a roux made with equal parts butter and flour, cooked until it's just golden before adding your milk. Gradually whisk in the milk to avoid lumps, and let it simmer until it reaches your desired thickness. You can also add a bit of cornstarch mixed with cold milk if you need extra thickening power.
start with a roux, which is just cooking equal parts flour and butter together until it’s bubbly, then whisking in some milk. If you prefer cornstarch, mix it with a bit of cold milk to make a slurry and stir it into the sauce, then cook until it thickens up.Another easy way is to add some heavy cream, which makes the sauce thicker and richer. If you’re into creamy sauces, cheese like Parmesan works great too—just melt it in.If you want to keep things interesting, you can also use pureed veggies like cauliflower or potatoes to thicken the sauce. Just blend them up and stir them in. Maybe this will help you too