Put the flour, ground nuts, confectioners' sugar and salt in the bowl of a food processor and pulse a couple of times to combine. Add the butter and pulse until the butter is cut in coarsely. Add the egg yolk and process in long pulses - about 10 seconds each - until the dough just starts forming clumps. Just before your reaches this clumpy stage, the sound of the machine working the dough will change. Wrap the dough in cling wrap and refrigerate until firm, about 2 hours.
For the Luscious Lemon Cream
Put the sugar and zest in a double boiler or a heatproof bowl over a pan of simmering water. Off the heat, rub the sugar and zest together between your fingers until the sugar is moist, grainy, and very aromatic. Whisk in the eggs, followed by the lemon juice.
Set the bowl over the pan and start stirring with the whisk. Cook the lemon cream until it reaches 180 degrees F. Whisk constantly until it's thick, opaque, and the whisk leaves tracks. This might take 10-15 minutes to get to 180 F.
As soon as it reaches 180F, remove the cream from the heat and strain it into the container of a blender. Let the cream stand, stirring occasionally, until it cools to 140 degrees F, about 10 minutes.
Turn the blender to high and, with the machine going, add the butter a few chunks at a time. Scrape down the sides of the container as needed as you incorporate the butter. Once the butter is in, keep the machine going for 3 minutes. Although it will look blended before then, this extended whipping will give it a light, mousse-like texture.
Pour the cream into a container, press a piece of plastic wrap against the surface to create an airtight seal and refrigerate at least 4 hours, or overnight. When you are ready to assemble the tart, just whisk the cream to loosen it and spoon it into the tart shell.
To assemble the tarts
Spray mini muffin tins with nonstick cooking spray and preheat the oven to 350 F. Dust a work station with flour and divide the tart dough in thirds. Keep two-thirds of it in the fridge and work with one third at a time. Roll it out very thinly on a floured surface with a floured rolling pin. Since we are making mini tarts the crust should be delicate and thin so it doesn't overwhelm the filling.
Use a large circular cutter to cut circles of tart dough--mine were about 3" wide. Gently lift the circles from the table and, one at a time, press them into the mini muffin cavities. Use a sharp knife or an offset spatula to trim the top and get a fairly smooth edge--it won't be perfect, but after they're baked they'll look fine. Freeze the first mini muffin tray while you form the second. If your dough gets too soft to work with, re-roll it and put it back in the freezer while you work with one of the chilled rounds in the fridge.
Once one of the muffin tins in the freezer is frozen, bake off the shells until they're light brown, about 10-12 minutes, rotating halfway throughout. Let them cool in the tins before attempting to remove, then gently slide them out and let them cool completely.
Fill with lemon cream (I found a pastry bag was easiest for this purpose) and top with season fruit or nuts.