For the filling: Toss the blueberries and nectarines in a large bowl with the lemon zest and juice. Add the rest of the ingredients and stir to combine. Divide the filling evenly among 4 (6-ounce) ramekins.
For the topping: Combine all of the ingredients in a food processor except the water. Pulse until combined, this will take about 30 seconds. Add the water, 1 tablespoon at a time, until the mixture is clumpy but crumbly.
Top each ramekin with the topping. Be sure to loosely sprinkle the topping and not pack it down. The idea is to look very crumbly and craggy.
Place the ramekins on a sheet tray and bake in the oven until the filling is hot and bubbly and the topping is brown and crispy, 20 to 25 minutes.
Sweeten the mascarpone with the sugar. Onto a small sheet pan or plate, neatly spoon 4 small portions of sweetened mascarpone and chill until ready to serve.
Top each crisp with a quenelle of cold mascarpone. Serve the crisps warm.