Cut the bacon into 1 inch pieces and slice the garlic clove. Meanwhile start boiling 4-5 quarts of water in a large pot for cooking pasta.
Discard the stem of shiitake mushrooms and slice them.
Discard the bottom of shimeji mushrooms and separate them by hands.
Heat the large frying pan on medium heat. Add the bacon and cook on both sides.
Add the garlic slices and cook until fragrant.
Add the mushrooms into the frying pan. Mushrooms absorb the oil fast. If necessary, add 1 Tbsp olive oil.
When the mushrooms are tender, add the spinach and saute. Season with freshly ground black pepper. Turn off the heat and set aside.
Add 1 Tbsp salt and pasta to the boiling water, stirring to make sure the pasta doesn’t stick to each other.
Cook the pasta according to the package instructions, but take out 1-2 minutes earlier since you will finish cooking in the frying pan. Reserve ¼ cup pasta cooking water before draining the pasta in a colander.
Put the frying pan back on the stove and turn on the heat to medium high heat. Add the pasta and butter to the pan. Combine all the ingredients. Add the pasta cooking water if you feel the pasta looks dry.
Season with freshly black pepper, add the soy sauce, and taste to see if you need it more. Serve immediately.