Shrimp: If shrimp frozen, defrost in cold (not warm or hot) water for 15-20 minutes. Drain and pat dry with paper towels before cooking. Make sure they are completely defrosted.
Heat pan: Heat your saute' pan on medium-high heat. If you're using a cast-iron skillet, medium heat may be hot enough to work with this recipe.
Add butter: Melt the butter.
Add oil: Once it's almost melted, add in the oil. If you do not want to use olive oil, you could cook the shrimp in just butter. It's up to you! Add in fresh lemon juice.
Season: Add in the salt and pepper. If using lemon slices, add them in. Add in crushed red pepper flakes, if using.
Add garlic: As soon as the butter and oil is simmering, add in the garlic and some fresh parsley (reserving some for serving).
Saute shrimp on one side: Add shrimp in a single layer keeping them separated so they cook evenly, and cook for 2 to 3 minutes until the shrimp turns pink.
Flip shrimp: Use a spatula to flip over the shrimp, and baste with the garlic sauce. Cook for 1-2 more minutes, or until turns pink and starts to start to curl into a "C" shape. Remove immediately from heat when other side is cooked, or it will get chewy.
Serve: Plate up and serve shrimp with the garlic butter sauce, a sprinkle of fresh chopped parsley, and lemon wedges.