Make a syrup by stirring the water, sugar, and lemon juice together in a saucepan over medium heat until thick; set aside to cool.
Make a filling by combining the ricotta cheese and cornstarch in a small bowl; set aside.
Make the dough by mixing together the flour and the yeast. Stir the water into the flour mixture 1 cup at a time, whisking to remove any lumps, until the mixture reaches a pourable consistency; allow to rest 1 hour.
Heat a skillet over medium heat. Ladle 2 to 3 ounces of batter at a time into the center of the skillet into a round shape. Each katayef shell is finished when the top is dry and the bottom is slightly browned.
Fill each shell with an equal portion of the cheese mixture. Fold one end of the shell over the cheese mixture and seal into a semi-circle shape by pinching along the edges.
Heat the frying oil in a small saucepan over medium heat. Fry the katayef in the oil until crispy and lightly browned; immerse in the syrup to coat; serve immediately.