Cook the Pasta: Fill a large pot with water, adding a generous amount of salt, and bring it to a rolling boil. Once boiling, introduce the spaghetti and cook it according to the package directions until it reaches an al dente texture. Before draining, save ½ cup of the starchy pasta water, then drain the spaghetti in a colander.

Cook Pancetta or Mushrooms: While the pasta cooks, heat olive oil in a large skillet over medium heat. Add the pancetta and cook until crispy, about 4–5 minutes. If using mushrooms, cook until tender and browned. Remove from heat and set aside.

Make the Sauce Base: In a medium bowl, whisk together the egg yolks, milk or cream, shredded parmesan cheese, miso paste, and minced garlic. Mix until the sauce is smooth and the miso is fully dissolved.

Combine Pasta and Sauce: Add the drained spaghetti to the skillet with the pancetta or mushrooms. Pour the sauce over the pasta, tossing to coat. Add a small amount of the reserved pasta water, stirring until the sauce reaches your desired consistency.

Serve and Garnish: Divide the miso carbonara into bowls and garnish with chopped chives, if desired. Serve immediately and enjoy!