Season the shrimp thoroughly with salt and pepper. Heat a large saute pan over medium-high heat. When the pan is hot, add enough oil to lightly coat the pan. Add the shrimp and quickly saute until just starting to turn pink, but not cooked through. Remove from the pan and set aside. Add the onions and saute just until they begin to soften, about 3 minutes. Add the garlic and cook another 30 seconds. Add the lemon juice, white wine, and stock, and reduce by 2/3, about 5 minutes. Add the shrimp back to the pan and swirl in the butter. Finish with the parsley and check for seasoning. Garnish with lemon slices and serve over rice.