Make the pasta dough: Mound the flour out onto your countertop. Create a well in the middle, and pour in the blended egg and oil. Using a fork, slowly whisk the eggs, incorporating more flour as you go. Alternatively, you can do this on low in the bowl of a stand mixer with the paddle attachment by slowly pouring in the egg. Be sure to slowly incorporate the flour into the egg so you can create a smooth, even dough. Once the flour is mostly all mixed in, use your hands to knead the dough until you have a mostly smooth and even ball, roughly 5 minutes. Wrap the dough in plastic and let it rest for a minimum of 30 minutes to allow the flour to fully hydrate and the gluten to relax.
Make the pasta filling: Place a sauté pan over low heat and add olive oil along with the diced onion. Season the onion with salt and sweat it until tender. Add garlic and sauté for another minute. Add the radicchio and sauté a few minutes more, until the radicchio is tender. Allow this mixture to cool slightly so you can pour it out onto a cutting board and chop it into smaller pieces. Combine the sautéed radicchio mixture with the ricotta, Parmesan, and black pepper in a bowl and mix, adjusting the seasoning as you wish.
Shape the pasta: Cut the dough into 4 pieces and roll it out until you can see the outline of your hand through the dough. Using a spoon or a piping bag, dot about 1 tbsp. of the radicchio-ricotta mixture, running down the middle of the dough, spaced about 2 inches apart. Fold the top half of the dough lengthwise toward you, over the ricotta and dust the dough with semolina. Working from left to right, slowly run your fingertips along the dough while pressing down to seal the pasta and work the air out of the filling. Use a knife or pasta cutter to cut the tortelli into squares. Place the shaped pasta onto a parchment-lined sheet pan dusted with semolina.
Assemble: Bring a medium pot of salted water to a boil. Place a saucepan over medium heat and add butter, toasted hazelnuts, and sage. When the butter starts to foam, stir with a wooden spoon or spatula, lightly scraping the bottom of the pan until the butter turns to a golden amber brown. Remove saucepan from heat and immediately add extra-virgin olive oil. Add radicchio and a pinch of salt and stir until just wilted. Drop your tortelli into the boiling water and cook 2 minutes. Using a slotted spoon, carefully scoop out the tortelli and add to the saucepan. It’s okay if some water makes its way into the pan! Adding a couple spoonfuls of the pasta water will help your butter sauce emulsify and become creamy. Finish the sauce with a squeeze of lemon juice. Spoon the tortelli and sauce onto a plate and garnish with grated parmesan and lemon zest.