Preheat the oven to 350 degrees F.
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.