Add 1½ Tbsp Diamond Crystal kosher salt and 4 quarts water to a large pot and bring it to a boil for cooking the spaghetti. Meanwhile, prepare the rest of the ingredients. Once the water is boiling, add 8 oz pasta and cook according to package instructions. Tip 1: Stop cooking 1 minute earlier as you will continue to cook the pasta in the frying pan. Tip 2: Before draining the pasta, reserve ½ cup (120 ml) pasta cooking water. Drain well and set aside.
Cut 4 slices bacon into ½-inch (1.25-cm) pieces.
Cut the bottom of 4 cremini mushrooms and slice them. Cut the bottom end of 1.8 oz shimeji mushrooms.
Remove the stems of 4 shiitake mushrooms and slice the caps.
In a large frying pan, heat 1½ Tbsp extra virgin olive oil on medium heat. Note: If you use a nonstick frying pan, you can skip the oil.
Once the oil is hot, add the bacon and sauté.
Once the bacon fat renders, crush 2 cloves garlic and add into the pan.
Add all the mushrooms and sauté together. Add 1 Tbsp unsalted butter and freshly ground black pepper.
Add 2 Tbsp all-purpose flour (plain flour) and make sure to keep stirring so the flour doesn’t stick at the bottom of the pan.
Stir in 1 cup milk, ⅓ cup heavy (whipping) cream, and 1 Tbsp soy sauce. Continue scraping off the bottom of the pan. The flour will thicken the sauce.
Taste the sauce and add ⅛ tsp Diamond Crystal kosher salt and freshly ground black pepper to taste. If the sauce is too thick, you can add ¼ cup of pasta cooking water (after that, add one tablespoon at a time) to dilute the sauce. Tip: You want to make sure it tastes a bit stronger than you want the final dish to be (because you will add the spaghetti).
Add the cooked spaghetti to the frying pan, or alternatively, you can pour the sauce over the spaghetti on a serving plate. Using the pair of tongs, coat the spaghetti with the sauce.
If you‘d like, add freshly ground black pepper. Serve and garnish with parsley.