Preheat the oven to 140oC/gas mark 1. Brush the base and sides of a 20cm x 30cm tin with a little melted butter, then line with non-stick baking paper and brush that with melted butter too.
Lay a sheet of filo pastry inside, pressing the edges up the inside of the tin, and brush all over with butter. Repeat the layers until all the pastry sheets have been added. Bake in the oven for 5 minutes, until the pastry is just beginning to crisp up but is still pale. Remove the pastry from the oven and allow any puffed-up parts to flatten down naturally.
Meanwhile, place the honey and lemon zest and juice in a large bowl. Split the vanilla pod in half, scrape out the seeds and add half of them along with the thyme leaves. Mix everything together well, then toss prepared fruit through the honey and lemon mix to coat.
Sprinkle the ground almonds evenly over the base of the pastry case and arrange the fruit in a single layer on top, reserving the juice for serving. Return to the oven to bake for 20 to 25 minutes, until the pastry is crisp and golden and the fruit has softened.
Meanwhile, mix the mascarpone, honey and reserved vanilla seeds together in a small bowl and refrigerate until ready to serve.
Lift the tart out onto a serving platter or board and brush the reserved juice over the fruit. Crumble over some amaretti biscuits, (if using). Dust with icing sugar, scatter the thyme sprigs over and serve at one with the vanilla mascarpone.