Melt the butter over medium low heat in a skillet large enough to hold the shrimp in a single layer. If you aren't doubling the recipe you don't need a very large pan.
Add the garlic and gochugaru. Cook gently for about 2 minutes. You don't want the garlic to take on any colour here. Think garlic butter.
Add the fish sauce and lemon juice. Keep cooking over low heat for 2-3 minutes. You want a gentle simmer.
Add the shrimp and continue cooking over low to medium low heat. GIve them 3 minutes. Again, gentle simmer. Little bubbles.
Flip the shrimp. If they are uniformly pink you are good. If you still see a bit of grey turn the heat up a smidge.
Cook the shrimp for another 3 minutes. If they were still a little grey when you turned them flip them and cook a minute more. You want just pink. Don't overcook your shrimp. Please don't overcook your shrimp.
Garnish with a little cilantro and serve with crusty bread to sop up the spicy garlic butter.