Place the mixed vegetables in a pot with enough water to cover and bring to a boil until the vegetables are tender, about 5 minutes. Drain and set aside.

Melt the butter in a pot over medium-low heat; cook and stir the onion, garlic, mushrooms, and Cajun seasoning in the melted butter until the onion is tender, but not yet browned, about 5 minutes. Stir in the flour. Pour in the milk; cook and stir until thickened and beginning to bubble. Add the scallops, salt, and pepper; continue cooking until the scallops are opaque, 5 to 7 minutes. Fold the vegetables into the mixture and cook until the vegetables are reheated, 2 to 3 minutes. Serve immediately.