In a small bowl, sprinkle the yeast over the lukewarm water. Stir briefly to combine, then allow to rest at room temperature until frothy, 5-10 minutes.
In the bowl of a stand mixer fitted with a dough hook or a large bowl, whisk together the flour, sugar, and salt.
Slowly mix in the frothy yeast with water, milk, and oil to bring together a dough. If too wet, add a little more flour until just soft enough to handle (do not add too much). If too dry, add a little more water at a time.
On a lightly floured surface, knead the dough until soft and elastic. Place in a bowl, cover, and allow to rest at room temperature until doubled, about 45 minutes to 1 hour.
Punch down the dough, then cover and allow to rest again for 45 minutes to 1 hour.
Preheat oven to 480˚F (250˚C). Grease 2 baking sheets lightly with oil.
On a lightly floured surface, divide the dough into 6 equal pieces. Form each piece into a smooth ball, seam side down. Cover and allow to rest at room temperature for 10 minutes.
Using your hands, press each ball down into a flattened round disc. Transfer to the prepared baking sheets and cover. Allow to rest for another 15 minutes.
Using the spine (dull side) of a knife, press lines across the disc-shaped dough to form a square or diamond pattern on top.
Bake in the preheated oven until golden, 8-12 minutes.
Remove from the oven. Immediately sprinkle the tops with water and wrap in a towel. This will keep the bread soft.
The Lepinje are best the day they are baked, but will last a couple of days at room temperature in an airtight container (after cooling).