In a large bowl, whisk flour, psyllium, yeast, sugar, and salt. Add water, olive oil, and vinegar; stir then knead in the bowl until a soft dough forms.

Turn onto a lightly floured parchment, shape into an 8x6-inch rectangle, roll up tightly, and stretch into a 14-inch log. Transfer to a baguette pan or baking sheet seam side down.

Cover and let rise in a warm place until puffy, 45 to 60 minutes. During the last 20 minutes, preheat oven to 450°F with a pan of water on the bottom rack and a stone or heavy sheet on the middle rack.

Slide loaf onto stone, pour hot water into the pan, and bake 25 to 30 minutes until deep golden and hollow-sounding.

Cool on a wire rack for at least 20 minutes, then slice with a serrated knife and serve warm.