Bring water up to a boil in a large pot.
Add in the udon noodles and cook until al dente, be sure not to overcook.
Drain the noodles and rinse under cold water. Set aside.
Heat a large pot or skillet over medium high heat and add in vegetable oil.
Add in the sliced mushrooms and sauté, stirring every so often, until the mushrooms cook down and most of the liquid has evaporated (leaving a little bit of liquid behind), about 7-8 minutes.
Next, add in the minced garlic and continue to stir until softened and fragrant.
Reduce the heat, transfer the drained udon noodles into the pot and add in 1 tablespoon of light soy sauce and 1 tablespoon of dark soy sauce.
Use a pair of tongs to coat the noodles in the sauce and season with black pepper.
Taste and adjust seasoning to your liking.
Remove from heat and drizzle in sesame oil.
Give the noodles a final toss and garnish with green onions and sesame seeds if desired.
Serve immediately.