Slice the bacon and mushrooms. Mince the garlic, and cut the spinach into about 3-inch pieces if needed.
Cook the spaghetti according to the package instructions. Reserve 1/2 cup of pasta water before draining.
Meanwhile, heat a pan over medium heat. Cook bacon until the fat is released.
Add the mushrooms, salt, and black pepper. Cook until the mushrooms become tender.
Add the garlic, chili flakes, spinach, spaghetti, butter, soy sauce, dashi powder, and pasta water.
Toss the pasta while shaking the pan until the sauce emulsifies and becomes glossy. Once the sauce coats the pasta evenly, serve immediately.
