First, make the pasta dough. Mound the flour on a large work surface, and using your fingers, form a well in the center. Add the eggs, egg yolk, olive oil, and salt into the well. Using a fork, lightly beat the wet mixture until combined, and then incorporate the flour, beginning with the inner edges of the well and working outwards until a shaggy dough forms. Using your hands, knead the dough until completely smooth, about 8 – 10 minutes. Wrap the dough in plastic wrap and allow to rest for at least 20 minutes. It is important to allow the dough to rest, otherwise you will find the gluten in the flour will make it very difficult to roll the pasta out. The dough can be made a couple of days in advance and kept in the fridge.
While the dough rests, prepare the filling. In a medium bowl, mix the ricotta, chopped walnuts, and grated Parmigiano until combined. Add a pinch of freshly ground nutmeg, and season with salt and pepper, to taste. Mix again and set aside. I like the walnuts very finely chopped so you can taste them but not feel them so much.
Once the pasta dough has rested, cut it into four equal pieces. Roll the first piece through a pasta machine on its widest setting. Decrease the width by one notch, and roll the dough through again. Continue in this method, decreasing the width by one notch at a time, until the pasta is very thin (ending somewhere between setting 5 and 7, depending on the make of your machine). Place the rolled sheet of pasta on a work surface dusted with flour, and repeat this process for the three remaining pieces of dough.
On one sheet of pasta, place rounded balls (about two tablespoons each) of the filling 1 to 1 1/2 inches apart. Brush the pasta around each filling ball lightly with water. Lay a second sheet of pasta on top and press firmly around each ball making sure there are no air pockets in the ravioli. Using a stamp cutter or pastry wheel, cut out each ravioli (about 2 inches in diameter) and transfer to a baking sheet lightly dusted with flour. Repeat this process with the remaining two sheets of pasta. I like to use a fork and press the edges of each ravioli to ensure the edges are closed and also to give them a pretty design.
Next, prepare the sauce. In a large skillet, melt the butter over medium heat. Add the minced garlic and fry until fragrant, about 1 – 2 minutes. Add the chopped walnuts and fry for an additional 2 minutes. Add the cream and mix well. Bring the mixture to a simmer and then reduce the heat to low. Allow sauce to simmer gently until slightly thickened, but still liquid, about 8 minutes. Season to taste with freshly ground black pepper.
While the sauce is cooking, bring a large pot of water to a slow boil. Carefully lower half of the ravioli into the water. Cook until the ravioli start to float in the water and are just tender, about 2 minutes. Using a slotted spoon, transfer the ravioli from the water to the sauce. Cook the remaining batch of ravioli in the same method and when done, also add them to the sauce. Sprinkle with the Parmigiano and gently toss to coat the ravioli in the sauce. Serve immediately.