In a tall, narrow container just wide enough to accommodate the head of an immersion blender, combine rolled oats and water. Using an immersion blender, purée until thick and smooth, then strain through a fine-mesh strainer, pressing on the solids with a flexible spatula until they give up all their liquid. Discard the fibrous oat solids and reserve 2 ounces of the thick and gooey purée to use as directed below. This step should not be done in advance, as the oat slurry will continue to thicken over time, eventually becoming unusable.
