Bring 2 quarts water to boil in large pot. Add 12 ounces spaghetti and ½ tablespoon salt and cook, stirring occasionally, until al dente. Reserve 1 cup cooking water, then drain pasta and return it to pot.
Meanwhile, cook 5 slices thick-cut bacon, sliced into ½-inch-wide strips, in 12-inch skillet over medium heat until bacon is golden and crispy, 6 to 8 minutes. Using slotted spoon, transfer bacon to paper towel–lined plate.
Add 1 pound assorted mushrooms, sliced ½-inch thick, to fat left in skillet. Cook over medium-high heat until mushrooms have softened and edges are golden, about 5 minutes.
Add 4 thinly sliced scallion whites and 3 thinly sliced garlic cloves and cook until fragrant, about 30 seconds.
Stir in ½ cup reserved cooking water, 5 tablespoons unsalted butter, 3 tablespoons soy sauce, and 1 tablespoon instant dashi powder.
Transfer to pot with pasta along with reserved bacon and toss to combine. Adjust consistency with additional reserved cooking water as needed.
Sprinkle with 4 thinly sliced scallion greens. Serve.
