1. Heat the oven to 300°F (150°C). Lightly butter a large rimmed baking sheet (approximately 18 x 13 inches / 46 x 33 cm).
2. In a large bowl, combine the fine oatmeal, rolled oats, flour, sugar, salt, and baking soda. Whisk together until evenly blended.
3. Melt the butter or lard in the hot water, stirring until fully dissolved. This step is a form of fat emulsification — incorporating the fat into the liquid before it meets the dry ingredients distributes it more evenly than cutting in cold fat. Pour the butter-water mixture into the dry ingredients and stir just until a stiff, cohesive dough forms.
4. Turn the dough out onto a well-floured board or work surface. Using your hands, pat and then roll the dough as thinly as possible — aim for 1/16 to 1/8 inch (1.5 to 3 mm). The dough will be firm; work in small sections if easier to manage.
5. Cut into rounds using a cookie cutter (approximately 2 to 2.5 inches / 5 to 6 cm), or use a sharp knife or pastry wheel to cut into strips roughly 1 x 3 inches (2.5 x 7.5 cm). Transfer to the prepared baking sheet.
6. Bake at 300°F (150°C) for 18 to 24 minutes, until the wafers are pale golden at the edges, completely dry to the touch, and snap cleanly when one is broken in half. They will crisp further as they cool. If wafers are very thin, begin checking at 15 minutes.
7. Transfer to a wire rack and cool completely before storing. Store in an airtight container at room temperature for up to one week.
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