Pudding vs. laundry starch - Add water gradually or all at once?
Yes, both processes are valid ways of making pudding (not roux). You can either dissolve the starch in a little cold water first and then gradually warm it up, or you can dissolve the starch in all of the cold water and then start warming it up. After it is warmed up, you can bring it to a boil.
The &quot;dump all together&quot; method is the more tedious one, because you have to stir constantly until you are ready, else the starch sediments and burns on the bottom, especially if you are an old-time cook using direct fire. If you warm up most in the water in a kettle first, your time of cooking the slurry (and stirring it) is reduced a lot, which makes it the preferred method.
Another reason is that most people cook blanchmange with milk, not with water. To reduce the amount of water used, they make the slurry with only a little water, then add the slurry to the warmed milk.