Start by making your pastry. Preheat the oven to 180C, then melt the butter in a saucepan. Stir into the ground almonds and salt until combined. Press the base into four small loose-bottomed tart tins, then bake for 8-12 minutes, or until golden. Set aside.
Preheat the oven to 180C. Toss together the filling ingredients - apart from the orange - then wrap in foil. Roast in the oven for 45-60 minutes, or until softened.
Sprinkle half of the feta over the tarts, then spoon over the beetroot. Place in the oven to bake for 10-15 minutes, or until the feta is melted and the pastry is golden brown.
Serve topped with more feta and mint.