In a food processor, pulse the walnuts, pecans, and pistachios until finely ground. Transfer to a large bowl and mix in the cinnamon and brown sugar. Set aside.

In a saucepan, combine the water, sugar, cinnamon stick, rose water, and lemon juice. Bring to a boil over medium-high heat, then reduce to a simmer and cook for 5 minutes. Remove from heat and let cool slightly.

Preheat the oven to 350°F (175°C).

Place a sheet of phyllo on a clean work surface and brush it generously with melted butter. Spoon a narrow strip of the nut filling along the long side. Fold the sheet in half lengthwise over the filling and roll into a log. Cut the log in half and coil each piece into a spiral.

Place the spirals in a well-buttered 9x13-inch/23x33 cm baking dish. Repeat with the remaining phyllo and filling. Brush the tops generously with more melted butter.

Bake for 30 to 35 minutes, or until the spirals are golden and crisp. Immediately pour the warm syrup evenly over the hot baklava. Let sit for 15 minutes to absorb.

Top with ground pistachios and rose petals before serving.