Make the syrup: put a pot with the water and sugar on the stove, stir and leave to come to a boil.
Smear the tray with a little of the butter, melt the rest.
Divide up the phyllo pastry sheets into 3 equal piles. Put 1 of the piles along the bottom of the tray, sprinkle with 1/2 the semolina and distribute 1/2 the walnuts.
Cover with the 2nd pile of sheets, sprinkle with the remaining semolina and walnuts and cover with the remaining sheets.
Take a sharp knife and cut the baklava into rhombuses, pieces or however you like.
Pour the melted butter over the baklava, distribute it so it covers the entire surface.
Put the tray in the oven and bake the baklava at 356°F (180 °C) until golden.
Pour the cold syrup over the ready baklava and leave in a cool area for it to take it in.