Toast the peppercorns in a dry skillet over medium heat for 1–2 minutes, until fragrant.

Grind to a coarse texture in a mortar and pestle, or use 1 tablespoon of freshly ground black pepper as an alternative.

Heat olive oil in a large skillet over medium-high heat. Add the pulled oyster mushrooms and maitake mushrooms in a single layer. Let them cook without stirring for 2–3 minutes to develop color, then stir occasionally until golden and tender. Lower the heat and stir in butter, allowing it to melt gently without browning. Remove the mushrooms, reserving the leftover butter.

Whisk together egg yolks, Pecorino Romano, miso paste, 3 tbsp of the reserved butter from the mushrooms, and the prepared black pepper in a heatproof bowl until smooth. Set aside.

Bring a large pot of salted water to a boil and cook the spaghetti until al dente. Reserve 1 cup of pasta water before draining.

Place the bowl with the egg mixture over the same pot of simmering water used for the pasta. Ensure the water is about 2 inches below the bottom of the bowl to prevent direct contact. If the water touches the bowl, carefully pour some out until the correct level is achieved. Add ¾ of the sautéed mushrooms, reserving the rest for garnish. Slowly add the reserved pasta water, one tablespoon at a time, while whisking constantly. Continue until the mixture is silky and clings to the back of a spoon without being runny. Remove from heat.

Add the drained pasta to the egg mixture, tossing quickly to coat the noodles evenly. Stir in the reserved oil from the mushrooms and adjust the consistency with additional pasta water if needed.

Serve immediately, garnishing with the reserved mushrooms, additional Pecorino Romano and freshly cracked black pepper.