How to Make It
Squeeze the lemon juice into a medium bowl of water; add the lemon. Working with 1 artichoke at a time, snap off the tough outer leaves. Using a sharp knife, cut off the yellow leaves at the top of the heart and all but 1 inch of the stem; trim the dark green stubs and fibers from the artichoke bottom and stem. Using a melon baller or spoon, scoop out the furry choke. Thinly slice the artichoke heart and put it in the acidulated water.
Drain the artichokes and put them in a saucepan. Add the heavy cream and garlic and cook over moderately low heat until the artichokes are tender and the cream thickens, about 15 minutes; add a few tablespoons of water if the cream gets too thick. Using a slotted spoon, transfer the artichokes to a food processor and puree until smooth. Transfer to a bowl and stir in the mascarpone, goat cheese and a generous pinch each of salt and pepper. Refrigerate until chilled.
In a food processor, pulse the flour, semolina, oil and salt to combine. Add the eggs and process just until the mixture resembles wet crumbs. Transfer the dough to a board and knead until silky, about 1 minute. Wrap the dough in plastic and let rest at room temperature for 30 minutes.
Cut the dough into 4 pieces. Using a hand-cranked pasta machine, roll 1 piece of the dough through successively narrower settings until you reach the thinnest setting. Lay the pasta sheet on a work surface and cover with a kitchen towel. Repeat with the remaining dough, layering it between towels. Using a 2 1/4-inch round biscuit cutter, cut out rounds from the dough sheets and arrange them without touching between layers of plastic wrap.
Working with 6 rounds at a time, brush each one lightly with water. Set a heaping 1/4 teaspoon of the filling in the center of each round and fold the dough over to form half-moons. Crimp and seal the edges, pressing out the air. Arrange the agnolotti in a single layer, without touching, on a baking sheet dusted with semolina. Continue making agnolotti with the remaining dough and filling.
In a medium nonstick saucepan, boil the Chicken Stock and thyme sprigs until reduced to 1/4 cup; discard the thyme. Reduce the heat to low and whisk in the butter. Season with salt and white pepper and remove from the heat.
In a large pot of boiling salted water, cook the agnolotti, stirring gently, until just al dente, about 2 minutes. Drain the agnolotti and add them to the saucepan, turning to coat with the sauce. Sprinkle with the Parmesan. Using a slotted spoon, transfer the agnolotti to 4 shallow soup plates and spoon any sauce on top. Garnish with the truffle shavings and serve.
Truffle oil is available at specialty food stores and by mail from Urbani Truffles, 718-433-1560.