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1. If using frozen wild berries (see note), thaw in a colander over a large bowl.
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2. Place 2 ounces of the berries and any reserved juice from the thaw in a tumbler and puree with an immersion blender. I use a wide tumbler that fits my blender, and set it inside a tall measuring cup for extra splash protection. Set aside.
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Prepare and measure all other ingredients. Mix the berry puree, lemon juice, vinegar, and honey in a cup measure. When things happen they will happen quickly, so have everything ready.
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3. Melt butter in 12-inch stainless steel skillet over medium heat, swirling constantly, until foaming. While using a wire whisk, sprinkle in flour. Whisk briskly and constantly to blend and cook, until the mixture begins to bubble. This will not take long.
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4. Add the prepared liquids and salt. Stir to combine. Add remaining blueberries and use a heat-proof spatula to stir until combined. Increase heat to medium-high and bring to a boil. Reduce to a low simmer and cook, stirring frequently and scraping down the sides of the pan, for about 15 minutes until the mixture is thickened and glossy. The time this step takes may vary due to the heat of your stove and how much liquid you got out of the berries.
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You will still see whole berries. In case it's not obvious at this point, that's a feature, not a bug.
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5. Take off heat and allow to cool for a few minutes. Season with salt (and pepper, if desired) to taste. Do taste it first. It may need very little salt, if any.
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Serve generously over a good steak, filet mignon if you want to be authentic.
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NOTE: Wild blueberries look and taste very different from domestic berries. They are smaller and have a more intense flavor. You can find them in the freezer section. Based on Cook's Illustrated's taste tests I buy Wyman's. The berries are excellent and widely available in supermarkets.
