Made with Flame Painter, in 4K (3840 x 2160).
For this one I started with the brush that drew the window, and made some lines. Then I experimented with the "eraser" tool, which is more of a "blurry eraser" -- it fades whatever it's above, you need to use it a lot to really erase a section. So I made the lines stop where they intersected.
Then I drew the "sky" and "grass" in separate layers, and used the eraser tool to remove what had bled into the window. Then I made the "portal landscape" and finished it up by glowing and blurring each layer except the one with the window panes in it.
Enjoy!