Mushroom Pappardelle with Radicchio

Directions

1. Fill a 5.5 quart Dutch oven with water, then bring it to a boil over high heat; season it very aggressively with kosher salt (figure a scant ¼ cup salt).
2. Meanwhile, trim the mushrooms, then tear into bite-size pieces. Cut the radicchio in half through the core, then cut away the core. Slice the radicchio into thin ribbons and transfer to a medium bowl. Cover the bowl with a damp kitchen towel or paper towel and refrigerate until ready to serve.
3. Thinly slice the garlic and cut the shallots into a small dice. Finely zest the lemon into a small bowl, then cut it in half and squeeze the juice into the same bowl. Grate enough Parm to yield ¼ cup (save what’s left for passing at the table). Set all aside.
4. Lower heat under the boiling water to maintain a simmer—you want to get your mushrooms going before starting the pasta. Heat a 10-inch cast iron skillet over medium-high for 1 minute, then add the olive oil and the mushrooms. Cook, tossing, until the mushrooms are coated with oil, then cook, undisturbed, until browned on the underside, 2–3 minutes.
5. Season with kosher salt and pepper and toss, then cook until lightly browned on the other side, 1–2 minutes more. Add the butter, tomato paste, thyme, garlic and shallot. Cook, stirring occasionally, until the mushrooms are starting to soften, the garlic and butter are golden brown and the shallot is translucent, 2–4 minutes. Turn off heat. (You can prepare the dish up to this stage several hours ahead of time; cover the skillet so that the mushroom mixture doesn’t dry out, and rewarm over medium before continuing.) Return the pot of water to a boil.
6. Add the pasta to the boiling water and cook, stirring occasionally, until pasta is very al dente, 2–3 minutes less than the time indicated on the package. Use a liquid measuring cup to scoop out 1½–2 cups of pasta water, then drain the pasta and return it to the Dutch oven. Add a splash of pasta water to the skillet with the mushrooms and use a wooden spoon to stir and scrape up any browned bits stuck to the pan. Scrape the mushroom mixture into the Dutch oven with the pasta. Add 1 cup of the pasta liquid and bring to a simmer over medium. Cook, tossing energetically with tongs, until a glossy sauce forms that coats the pasta, 2 minutes. Add the grated Parm, lemon juice mixture and another big splash of pasta water and cook, tossing, until cheese is melted and the sauce is clinging to the noodles, 1–2 minutes more. Pluck out thyme sprig.
7. Add balsamic vinegar to the radicchio. Drizzle lightly with olive oil and season with flaky salt; toss to combine. Divide pasta equally among 6 warm bowls and top each with some of the radicchio salad. Pass more cheese at the table.