Prep the pan: Grease a 9x5-inch loaf pan and line it with a parchment sling for easy removal.
Set aside.
Mix the dry ingredients: In a large bowl, whisk the gluten-free flour blend, tapioca starch, yeast, sugar, salt, and xanthan gum (if needed) until well combined.
Combine the wet ingredients: In a separate bowl or measuring jug, whisk the warm milk, warm water, oil, eggs, and vinegar until smooth.
Make the dough batter: Pour the wet mixture into the dry mixture. Beat with a hand mixer or stand mixer (paddle attachment) on medium for 2–3 minutes. The texture should be like thick cake batter—spreadable, not kneadable.
Pan and smooth: Scrape the batter into the prepared pan.
Smooth the top with a damp spatula. If using seeds, sprinkle them on now and lightly press.
Rise: Cover loosely with a clean towel. Let rise in a warm, draft-free spot until the dough crowns about 1/2 inch above the rim, 30–45 minutes.
Do not overproof.
Preheat the oven: While the loaf rises, preheat to 350°F (175°C).
Bake: Bake 40–50 minutes, until the top is golden and the internal temperature reaches about 205–210°F. If the top browns too quickly, tent with foil for the last 10–15 minutes.
Cool properly: Let the bread cool in the pan for 10 minutes, then lift it out and cool completely on a rack, at least 1 hour. Slicing too soon can make it gummy.
Slice and enjoy: Use a serrated knife to cut even slices.
For thinner slices, chill the loaf for 30 minutes first to firm it up.