Shepherd’s pie, a traditional main dish, is made with a mashed potato crust and a hearty meat and gravy filling. Most recipes call for flour when making the gravy for the filling. You could use cornstarch instead of flour, though it is more suitable for fruit fillings in sweet pies.
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As a thickener, cornstarch imparts little flavor. Once dissolved, you’ll hardly know it’s there. Flour, on the other hand, can have a starchy taste unless it’s cooked for several minutes, at which point it lends itself to a smooth, velvety texture. Recipes for shepherd’s pie require the meat filling to be cooked before baking, so the raw taste of flour is not a problem here.
The beauty of cornstarch is that it cooks down clear, giving sauces and fruit fillings a glossy sheen. This is desirable when making fillings for baked goods, but can make gravies look strange. Gravy thickened with flour for a shepherd’s pie will be opaque.
Cornstarch has twice the thickening power of flour. When using cornstarch as the thickener for the gravy in shepherd’s pie, use one-half the amount of flour called for in the recipe. For example, Betty Crocker’s recipe for shepherd’s pie calls for 2 tablespoons of all-purpose flour. When using cornstarch, halve this amount and use 1 tablespoon of cornstarch.
Cornstarch is known for breaking down if exposed for too long to heat. The taste of flour only improves with cooking, although flour can lose its thickening power if the dish is frozen and then reheated. When using cornstarch to thicken a shepherd’s pie, there is no need to cook the gravy several minutes before baking. Combine the cornstarch with a bit of cool liquid first, such as water or broth, slowly stir it into your meat and vegetables until thickened, then transfer to your casserole dish and bake immediately.
Cornstarch can lose its thickening power when mixed with too many acidic liquids. Some recipes for shepherd’s pie call for tomato paste or tomato sauce as an ingredient in the gravy. You might consider omitting or halving the amount of tomato if you're using cornstarch. Most recipes also ask you to blend the flour with a bit of melted butter or fat, as this improves the taste when thickening with flour. If you're using cornstarch, you could opt for a low-fat version as the butter isn’t necessary when thickening with cornstarch.