How to use icons with MenuFlyoutItem in Windows App SDK (Microsoft.UI.Xaml.Controls, WinUI3)

Conclusion Use <MenuFlyoutItem.Icon>

Reference: MenuFlyoutItem.Icon property

Example

<MenuBar>
    <MenuBarItem Title="File" x:Uid="File">
        <MenuFlyoutItem x:Name="Open" Text="Open" x:Uid="Open">
            <MenuFlyoutItem.Icon>
                <FontIcon Glyph="&#xED25;" FontFamily="Segoe MDL2 Assets" />
            </MenuFlyoutItem.Icon>
            <MenuFlyoutItem.KeyboardAccelerators>
                <KeyboardAccelerator Modifiers="Control" Key="O"/>
            </MenuFlyoutItem.KeyboardAccelerators>