Source: Mrs Beeton's Book of Household Management (1861)
Slice the onions into thin half-moons — roughly 3mm — and place them in a heavy-bottomed pot with the butter over medium heat. The onions will release their moisture and soften within 5 minutes, but don't stop there. Continue cooking for 20–25 minutes, stirring occasionally, until they turn a deep golden brown with caramelised edges. This onion transformation is non-negotiable: the sugars in the onions undergo the Maillard reaction, developing the savoury depth that makes this soup worth eating rather than merely filling.
While the onions cook, mix the rice flour with a little cold water to form a smooth paste, then whisk it into the remaining water until no lumps remain. This suspension prevents the starch from clumping when it hits the hot stock. Once your onions have coloured, pour the rice flour mixture directly into the pot along with a pinch of salt and pepper. The rice flour acts as a thickening agent — its starch granules absorb liquid and swell under heat, creating a light, slightly viscous body without the greasiness of flour-based roux. Add the powdered sugar now; a teaspoon cuts the natural bitterness of deeply caramelised onions and is a period-appropriate move.
Bring the liquid to a simmer and maintain a gentle rolling boil for 15 minutes, stirring occasionally, until the onions are completely tender and the rice flour has fully hydrated and thickened the soup. The surface should coat a spoon lightly but not cling to it — soup, not slop.
Finish with a beurre manié: knead equal parts softened butter and plain flour together to a paste, then whisk it in a small handful at a time until the soup reaches the consistency you want. This secondary thickening is optional but traditional; it adds richness and gives you final control over body. Taste and adjust seasoning — onion soup needs more salt than you'd expect — then serve hot in warmed bowls.
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