Place the cashews in a food processor or nut grinder and process until they form a smooth, fine powder. Set aside for later use in the sauce.
Heat the oil in a large frying pan over medium-high heat. Add the garlic and scallops and stir-fry for 2 minutes, just until the scallops become opaque and slightly firm. Be careful not to overcook them. Remove the scallops with a slotted spoon and set aside on a plate.
In the same pan, add the onion, green bell pepper, and chile. Reduce the heat to medium-low and cook gently, stirring occasionally, for 3–5 minutes until the onion softens and becomes translucent without browning. Sprinkle in the turmeric and stir continuously for 1 minute to release its earthy aroma and vibrant color.
Add the ground cashews and stock to the vegetables in the pan, stirring well to combine everything into a smooth mixture. Bring to a boil, then reduce the heat and let the sauce simmer for 5–10 minutes, stirring occasionally, until it thickens to a creamy, coating consistency.
Gently return the cooked scallops to the pan along with any accumulated juices. Stir in the mustard and season with salt and pepper to taste. Warm everything through over low heat for just a minute or two, being careful not to overcook the scallops. Serve hot over rice or with crusty bread to soak up the delicious sauce.