We present a novel interactive design tool that allows users to create and visualize gallery walls via a mixed reality device. To use our tool, a user selects a wall to decorate and chooses a focal art item. Our tool then helps the user complete their design by optionally recommending additional art items or automatically completing both the selection and placement of additional art items. Our tool holistically considers common design criteria such as alignment, color, and style compatibility in the synthesis of a gallery wall. Through a mixed reality device, such as a Magic Leap One headset, the user can instantly visualize the gallery wall design in situ and can interactively modify the design in collaboration with our tool's suggestion engine. We describe the suggestion engine and its adaptability to users with different design goals. We also evaluate our mixedreality-based tool for creating gallery wall designs and compare it with a 2D interface, providing insights for devising mixed reality interior design applications.