1. Make the water roux

• In a small saucepan, whisk together all roux ingredients until smooth.

• Cook over medium-high heat, whisking constantly, until it thickens enough that the whisk leaves clear lines in the mixture.

• Remove from the heat right away and let it cool to warm room temperature.

• You can prepare the roux ahead of time and refrigerate it in an airtight container. Just bring it back to room temperature before using.

2. Prepare the dough

• Grease and line a 9×5-inch loaf pan; set aside.

• In the bowl of a stand mixer fitted with the paddle attachment, add 3 cups flour, xanthan gum, cream of tartar, sugar, and yeast. Whisk these dry ingredients together by hand.

• Add the salt and whisk again.

• Pour in the vinegar, egg, milk, butter, and cooled water roux. Mix on low until combined, then increase to high speed and beat for about 3 minutes. The dough will look shaggy and feel tacky.

3. Shape the dough

• Lightly dust your work surface with flour and turn the dough out.

• Divide the dough into 3 equal portions (about 350 g each).

• For each portion, gently pat it into a disk about ¾-inch thick, then fold it loosely in half from short end to short end.

• Place the folded pieces in the loaf pan, lined up snugly from end to end with the folded (rounded) sides facing upward.

• Lightly mist the dough with warm water, cover loosely with plastic wrap, and let it rise in a warm, draft-free place until it reaches about 150% of its original size.

4. Bake the bread

• While the dough rises, preheat the oven to 350°F (175°C).

• Once risen, remove the plastic wrap, brush the top with melted butter if using, and bake for 45 minutes.

• Carefully remove the loaf from the pan and place it on a rimmed baking sheet. Return it to the oven.

• Bake for about 10 minutes more, or until the loaf sounds hollow when tapped and reaches 190°F (88°C) internally.

• Transfer to a wire rack and let the bread cool completely before slicing.