Start by heating the olive oil in a large sauté pan over medium-high heat. Once hot, add the mixed sliced mushrooms. Cook, stirring occasionally, until the mushrooms release their moisture and become golden brown—about 8 to 10 minutes.
While the mushrooms cook, bring a medium pot of water to boil. Add the soba noodles and cook according to package instructions, usually around 5 to 7 minutes. Drain and rinse the noodles under cold water to stop the cooking process and prevent sticking.
To the browned mushrooms in the skillet, pour in the vegetable broth and water. Stir in the soy sauce and bring the mixture to a gentle simmer. Let it cook for 5 minutes to marry the flavors and warm the broth completely.
Turn off the heat and stir in the sesame oil for a fragrant finish. Season the broth with salt and pepper to taste. Add the chopped scallions, reserving a few for garnish.
Divide the soba noodles between serving bowls and ladle the hot mushroom broth over the noodles. Sprinkle the remaining scallions on top for a fresh burst of flavor and a beautiful presentation. Enjoy immediately.
