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Whisk the two ingredients together in a small bowl until the cornstarch dissolves, forming a thick slurry. Set the dish aside. Cornstarch and flour are both thickeners, but cornstarch also gives thickened liquids a mild gloss, while flour alone can leave gravies looking dull. Since cornstarch is sensitive to prolonged cooking, however, you'll need to use both cornstarch and flour as thickeners in this recipe. Toss the butter into a large saucepan and set it over medium to medium-high heat. Let the butter melt completely. Cutting the butter into smaller pieces prior to adding it may help the butter melt faster. Spread the melted butter over the entire bottom of the pan by shifting the pan as it melts. Toss the chopped shallots into the melted butter. Cook, stirring frequently, for 30 to 60 seconds or until the shallots become notably fragrant. Shallots have a mild, sweet onion taste that is also somewhat reminiscent of garlic. If you don't have any, you can use 1/4 cup (60 ml) chopped sweet onion or 2 large, minced garlic cloves. You may also omit the ingredient completely, but doing so will result in less flavorful gravy. Sprinkle the flour over the contents of the pan. Cook, whisking continually, for about 5 minutes or until the roux looks golden brown. You need to cook the flour roux for at least 1 minute after the flour absorbs the remaining butter; otherwise, the gravy may retain some bitterness from the taste of raw flour. Pour the broth into the seasoned roux, whisking constantly to combine. Bring the mixture to a boil while you continue whisking. You can use nearly any broth for this recipe, including beef, chicken, or turkey. To create a generic brown gravy, consider using two parts beef gravy and one part chicken gravy. Once the gravy is at a boil, pour in the cornstarch slurry and whisk well. Continue simmering the gravy for another 3 to 5 minutes, or until it thickens. Note that the water and cornstarch may separate as the slurry sits, so you may need to quickly stir the slurry before mixing it into the gravy. Sprinkle the gravy with black pepper, as desired, and mix to combine. Most commercial broths are fairly salty, so you probably won't need to add any extra salt. If you choose to do so anyway, add the salt in small amounts and taste the gravy in between each addition. For the best flavor and texture, serve the gravy immediately, while it's still hot. You can keep the gravy warm by setting it on the stove over very low heat, but try to use it within the next 15 to 20 minutes. Cornstarch can break down when exposed to prolonged heat, and if that happens, the gravy will become thinner. Refrigerate any leftovers and reheat them on the stove prior to use. Whisk the gravy well to work out any congealed lumps.

Summary:
Combine the water and cornstarch. Melt the butter. Add the shallot. Whisk in the flour. Gradually pour in the broth. Whisk in the cornstarch slurry. Season the gravy. Serve.