Trim off the bottom 1 to 2 inches of the enoki mushroom cluster where the stems are connected and discard this spongy base. Gently separate the mushrooms into smaller bunches, pulling them apart with your fingers to create clusters of 10 to 15 stems each. Rinse them briefly under cold water if they seem dirty, but avoid soaking them.

Combine the soy sauce, sesame oil, and black pepper in a small bowl and whisk together until well blended. Set this bowl within easy reach of your stove.

Place your skillet over medium-high heat and let it warm for about 1 minute until hot. Add the butter and swirl the pan to coat the bottom evenly as it melts and starts to foam. Wait until the butter stops foaming and just begins to turn golden brown around the edges.

Add the minced garlic to the browned butter and stir constantly for about 30 seconds until fragrant and just beginning to turn golden. Watch carefully to prevent burning.

Add the separated enoki mushrooms to the pan, spreading them out in a relatively even layer. Let them sit undisturbed for about 2 minutes to develop golden, crispy bottoms.

Use tongs or a spatula to flip and stir the mushrooms, exposing new surfaces to the hot pan. Continue cooking for another 3 to 4 minutes, stirring occasionally, until the mushrooms are golden brown in spots and have reduced in volume by about half.

Pour the soy sauce mixture over the cooked mushrooms and toss everything together quickly to coat evenly. Cook for just 30 seconds to 1 minute more, stirring constantly, until the sauce has mostly evaporated and clings to the mushrooms.

Remove the pan from heat immediately and transfer the mushrooms to a serving plate. Sprinkle with chopped green onions and red pepper flakes if using. Serve immediately while hot and crispy.