Force aspect ratio with crop on external monitor while mirroring display

When I connect my Macbook Pro to an external monitor with a different aspect ratio than 16:10 (which the Macbook Pro has) and let it mirror my display, the resolution is automatically changed to something that fits the external monitor. Also, my MacBook Pro screen is cropped.

Can I change this around? I want to force a certain aspect ratio, and force the external monitor to crop its display.