Thaw the phyllo dough according to the package instructions.

Preheat the oven to 350°F (180°C, or gas mark 4).

Rub a small amount of melted butter onto the bottom and sides of a 9 × 13-inch glass baking dish.

Place the pecans, walnuts, and almonds in a resealable plastic bag.

Use a rolling pin or kitchen mallet to crush the nuts into small pieces.

Add cinnamon to the bag, then shake to combine.

Cut the stack of phyllo dough in half to fit the dish.

Place 2 sheets of phyllo at the bottom of the dish and brush with melted butter.

Repeat this process until you have 8 layers on the bottom.

Sprinkle a layer of the nut mixture evenly over the phyllo.

Cover with 2 more sheets of phyllo and brush with butter.

Continue layering phyllo and nuts until 6–8 sheets of phyllo remain.

Use the remaining phyllo sheets to create 6–8 final layers on top, brushing with butter after every 2 sheets.

Ensure the top is fully covered and brushed with butter.

Using a very sharp knife, cut the baklava into 12 equal squares, then cut each square into 2 triangles for the traditional shape.

Bake the baklava for 45–50 minutes, or until the top is golden brown and flaky.

While the baklava bakes, prepare the syrup by boiling the water and sugar in a saucepot until the sugar dissolves completely.

Reduce to a simmer and add agave nectar and vanilla.

Simmer for 20 minutes.

Once the baklava is out of the oven, immediately drizzle the syrup evenly over the top.

Allow the dessert to absorb the syrup as it cools.

Let the baklava cool completely before serving.

Avoid covering it, as this may make the phyllo soggy.