Coarsely grind the pistachios in a food processor with the sugar, pulsing rather than running it. Remove 40 g, grind it finer, and set aside for the garnish.
Unroll one package of phyllo and lay the sheets flat. Set the pan upside down on top and cut around it so every sheet matches the pan. Return the trimmed stack to its plastic sleeve. Repeat with the second package. Save the trimmings for another use.
Gently melt 200 g of butter without letting it brown.
Butter the pan thoroughly with the remaining 50 g of soft butter.
Lay the first package of trimmed sheets into the pan one at a time, brushing each with melted butter.
Spread the ground pistachios over the surface in an even layer.
Lay the second package of trimmed sheets on top, brushing each with melted butter. Tuck the edges down inside the pan and pour any remaining butter over.
Refrigerate for 30 minutes, until the butter is firm.
Cut all the way through to the base of the pan, into diamonds, using a long sharp knife.
Mist the surface lightly with water.
Bake at 170°C / 340°F on the middle rack for 30 minutes, until golden.
Reduce the oven to 150°C / 300°F. Meanwhile, bring the sugar, water, and lemon slices to the boil.
Remove the baklava. Pour off any pooled butter.
Slowly pour the hot syrup over the hot baklava, a ladleful at a time, letting each addition absorb before adding more.
Scatter the reserved ground pistachio over each diamond.
Cool completely, uncovered and out of the fridge, before serving. Best on the second or third day.