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Heat oven to 400°F. Line a sheet pan with parchment paper.
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Lay the puff pastry directly onto the parchment. With a knife, lightly score a border about 3/4 inch in from the edge, being careful not to cut all the way through.
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Drizzle about 2 tbsp honey over the area inside the scored border and spread it into a thin layer using the back of a spoon.
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Arrange the sliced plums over the honey, overlapping them slightly. Leave the scored border uncovered.
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Sprinkle the plums directly with the sugar, mixed spice, and a small pinch of salt.
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If using, brush the exposed pastry border lightly with milk or cream.
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Bake for 25–30 minutes, until the pastry is deeply golden and puffed and the plums are soft and slightly caramelized.
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Remove from the oven and immediately drizzle with a little more honey.
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Scatter the chopped pistachios over the tart.
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Let cool for about 10 minutes before slicing.
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Serve warm or at room temperature.
