Display on an HP 23fi monitor with a MacBook Pro

I've recently started using a Mac. I've been on Debian + Thinkpads exclusively till now.

I have an HP Pavillion 23fi monitor which I've been using with my old Thinkpad X201 via the VGA port. It drives the monitor at 1920×1080 (60.0 Hz) and everything looks sharp and nice.

Now that I have the retina MacbookPro, I've connected it to the monitor using a Belkin HDMI to HDMI cable. It looks symmetric. I've put one end into the macbooks HDMI port and the other end into the monitors HDMI port. The MacBook detects it fine (understands that it's is a HP 23fi) and I can see stuff on the screen.

Now, the problems start. I want the laptop retina screen to be turned off and the external HP monitor to be driver at 1920×1080 (60.0 Hz). If both the laptops are plugged in, I can simply change the input terminal on the monitor and use them both conveniently. With Debian, I run this on the terminal and everything works fine.

xrandr --output VGA1 --mode 1920x1080

which basically tells the external monitor (VGA1) to run at 1920×1080. After that, I usually run

xrandr --output LVDS1 --off

which shuts off my laptop screen.

I want to do something like this with the mac but have hit some problems. I can set the built-in retina display to say "Best for HP 23fi" under system preferences. With the HP23fi, I set "Resolution" to "Best for HP 23fi", Rotation to "standard" , Refresh rate to "60 Hz (NTSC)" and underscan to "more" (without which, I can't see the entire screen).

This "works" but the fonts on the screen are blurry and I can't really use anything. I'm not familiar with how the retina display affects things but I'd like the external monitor to be driven at top resolution and get output similar what I'm getting with the Debian laptop. How exactly do I do this?

There might be bits of relevant information I'm missing since I'm not familiar with the platform.