Melt butter in a large skillet over medium heat. Saute the garlic until fragrant. Add bacon; cook and stir until lightly browned. Then add mushrooms; stir until mushrooms are tender and lightly browned, about 5 minutes.
In the meantime, bring a large pot of lightly salted water to a boil. Cook spaghetti in the boiling water, stirring occasionally until cooked through but firm to the bite (about 30 sec to 1 min less than the total recommended cooking time on the package.) Drain, reserving 1 cup of the pasta water.
Pour reserved pasta water and drained spaghetti into the mushroom-bacon mixture; stir to combine.
Stir in dashi powder, soy sauce and black pepper; cook until flavors combine and water evaporates, about 1-2 minutes. Add salt and pepper to taste.
Toss the green onions into the pasta before serving or serve with green onions on top.
