Fill a medium pot with warm water. Attach the sous vide and set the temperature to 123 degrees F (51 degrees C) to preheat.
Remove tough muscle from sides of scallops, if present, and lightly season with salt and pepper. Transfer scallops to a resealable plastic bag and remove all the air using the water immersion method.
Place scallops into the preheated water in the pot and cook for 30 minutes.
Remove scallops from the bag and pat dry thoroughly; if scallops are wet, they will not sear well.
Heat olive oil in a small skillet over medium-high heat until it shimmers. Sear scallops in hot oil until bottoms are lightly brown, about 2 minutes. Turn and cook for 1 more minute. Remove scallops from the skillet to a warmed dish.
Add garlic to the skillet; cook and stir until fragrant, about 30 seconds. Deglaze the skillet with white wine and lemon juice, scraping up any browned bits. Mix in parsley and Cajun seasoning; simmer sauce for 2 minutes. Stir in butter until just melted.
Spoon sauce over scallops; sprinkle with green onion tops to serve.