Start by preparing the lemongrass by peeling off the outer few layers and cutting off the leafy top half of the stalk. Thinly slice the white stalky portion of the remaining stalk.
Combine the lemongrass slices with the garlic cloves and chili in a mortar pestle and grind until you get a rough paste. If you don't have a mortal and pestle, you can mince all the ingredients and combine them, but I would highly recommend getting one as the flavor you can create from it is unreal. Also it's just fun to use!
Gather the shrimp & marinade ingredients, including the lemongrass chili paste. Combine them in a bowl and let marinade for 10-15 minutes.
While the shrimp is marinading, start putting together the rest of the dish. Cook the udon noodles and set aside. Mince the remaining garlic cloves, then melt the butter in a skillet.
Add in the onions and sliced mushrooms and sauté in the butter, with cooking spray if needed, until the garlic is slightly golden. Make sure to constantly stir so the garlic doesn't get burnt.
Begin to add in the shrimp into the skillet in a single layer and sauté for about 1-2 minutes on each side, or until the shrimp begins to curl.
Mix in the cooked udon noodles, fish sauce, and oyster sauce, and stir thoroughly until well mixed. Then begin to mix in the parmesan cheese.
Garnish with cilantro and enjoy your delicious garlic shrimp noodles!