In a small skillet, toast the pine nuts just until fragrant, moving them around in the pan to avoid burning for about 2 minutes. Remove and set aside.
In a food processor or blender, pulse the basil, pine nuts, salt, and pepper until finely chopped. With the food processor or blender running, gradually add enough of the olive oil to form a smooth consistency. Add in the grated parmesan cheese just until mixed through.
The pesto can last in the refrigerator for 2 days. Just cover tightly and refrigerate.
Bring water to a boil in a large pot. Once boiling, generously add salt to the water and add in the pasta. Stir every few minutes until cooked 'al Dente', about 8 minutes. Before draining, scoop out about 1 cup of the cooked pasta water and set aside.
While the water is boiling, heat the 2 tablespoons of olive oil in a large, deep skillet. Once the oil is glistening (hot), add in the sliced mushrooms. Cook over medium-high heat, stirring frequently. After about 3 minutes of cooking, season with salt and pepper. If the mushrooms are cooking too fast, lower the heat. You can ladle in some of the pasta water to scrape up any bits from the pan, adding more flavor from the mushrooms.
Once the pasta is cooked, drain and add it to the mushroom mixture, gently combining with a large spoon. Turn off the heat. Start adding pesto sauce in 3 tablespoons at a time. You don't want it too saucy, just enough to add flavor. Add in about 1/4 cup of the reserved pasta water and carefully mix through to help coat the pasta beautifully.
If you prefer a thinner sauce, add in more pasta water to achieve the desired consistency.
Plate the pasta into bowls and sprinkle with more grated parmesan cheese. Enjoy!