To make the filling, preheat oven to 180°C. Spread the hazelnuts and walnuts on a baking tray and lightly toast in the preheated oven for 5-8 minutes or until aromatic. Then leave to cool.
Combine walnuts, hazelnuts, pistachios, sugar and cinnamon in the bowl with a food processor using the pulse button until they are finely chopped.
Remove the filo pastry from its packet and lie flat on the work bench. Cover with a dry tea towel and then a damp tea towel. (This will keep the filo from drying out while making the baklava.)
Brush a shallow 18 x 28cm cake tin with some of the butter. Take 1/3 of the sheets of filo. Brush the top sheet generously with butter and fold into thirds to make a rectangle the size of the tin. Place in the base of the buttered tin and brush surface with butter.
Continue layering with the left over 1/3 of pastry. Spread 1/2 of the nut filling over the filo to cover. Layer 1/2 of the remaining filo sheets as before. Spread leftover nut filling over filo and top with the remaining layered buttered filo sheets. Lightly brush top with remaining butter.
Using a sharp knife, cut a diamond pattern into the top layer of filo and sprinkle with the water. Bake in preheated oven for 20 minutes. Cover with foil and cook for 45 minutes longer or until the filo layers are cooked.
Meanwhile, to make the honey syrup: combine honey, sugar, water, thyme, lemon rind and juice in a saucepan and stir over high heat until sugar dissolves. Bring to the boil and maintain over medium heat for 10 minutes or until the syrup has thickened slightly.
Remove from heat and allow to cool. When cooked, remove baklava from the oven and immediately pour the cooled syrup evenly over the surface. Stand to cool completely. Cover with foil and store at room temperature in the tin for up to 2 weeks. Cut into diamond shapes to serve.