Preheat the oven to 200° (392 F)
Whisk the eggs in a mixing bowl. Add the wet ingredients (milk, sparkling water, and oil) and mix it all quickly together. Then, crumble some feta cheese and stir it lightly into the dough.
In another bowl, add and mix the dry ingredients (cornflour, all purpose flour, baking powder, and a little bit of salt).
Now add the dry ingredients to the bowl with the wet ingredients and give it all a good stir. Whisk or stir until you're left with a homogeneous, runny, and bubbly consistency without any lumps in the dough.
Add a little sunflower oil to your baking dish of choice and sprinkle and dust a little flour all over the dish, until it is coated very lightly.
Pour the mixture into the square baking dish (our measures 27cm x 17 cm) previously greased.
Bake for 20 minutes and then have a look at the proja. If it's getting way too dark too quickly, you can lower the oven temperature to 180° and bake for 10-20 additional minutes.
After 30 minutes of baking, insert a toothpick into the proja. If it is dry, it is fully baked. If not, continue baking for 5 minutes. You'll know that it's close to being done when the top is golden and kind of caramelized.
Cut the proja into 8 cm (3-inch) squares to serve.
Eat it straight away as a breakfast served with yogurt, soft cheeses, ajvar, and some fresh greens, or enjoy it as a side dish alongside with dishes like prebranac and the like.