Bring a large pot of salted water to a boil and cook the tagliatelle according to package directions until al dente.
While the pasta cooks, season the chicken breast with half of the sea salt and black pepper.
Heat the olive oil in a large skillet over medium-high heat and sear the chicken until golden and cooked through, about 5-6 minutes per side. Remove from the pan and slice into strips.
In the same skillet, add the sliced cremini mushrooms and sauté until they release their moisture and turn golden brown.
Add the minced garlic and baby spinach to the mushrooms, sautéing for 1 minute until the spinach is just wilted.
Drain the pasta, reserving 2 tablespoons of pasta water, and add the noodles to the skillet with the vegetables and chicken.
Toss everything together with the reserved pasta water, truffle oil, and the remaining salt and pepper.
Serve immediately topped with freshly grated parmesan cheese.