Gather Your Ingredients
12 ounce spaghetti
Table salt for cooking pasta
5 slices thick cut bacon, cut crosswise into ½-inch strips
1 pound assorted mushrooms, sliced ½-inch thick
4 scallions, white and green parts separated and sliced thin
3 garlic cloves, sliced thin
5 tablespoons unsalted butter
3 tablespoons soy sauce
3 teaspoons dashi powder
Bring 2 quarts water to boil in large pot. Add pasta 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 bacon 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 mushrooms to fat left in skillet. Cook over medium-high heat until mushrooms have softened and edges are golden, about 5 minutes. Add scallion whites and garlic and cook until fragrant and butter is melted, about 30 seconds. Stir in ½ cup reserved cooking water, butter, soy sauce, and 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 scallion greens. Serve.