1) Stir the flour and sugars in a medium bowl to blend. Add the biscuits and almonds and mix well. Add the butter and rub in until moist clumps form.
2) Preheat the oven to 180C/Gas 4. Butter a 33 by 22 by 5-cm glass baking dish. Stir the sugar and flour in a large bowl. Add the nectarines and blueberries, and toss to combine. Stir in the liqueur.
3) Spoon the fruit mixture into the prepared dish. Sprinkle the biscuit topping over. Bake until the nectarines are tender and the topping is golden and crisp, about 45 minutes. Cool at least 10 minutes. Spoon the warm crisp into bowls. Top each with a dollop of mascarpone cheese and serve.