Help I deleted my operating system!

Hi all,

I am trying to upgrade my Mid-2012 Macbook Pro so I started by erasing my hard drive.

I am a noob so I messed up and didn't realize it would delete my OS also. I was on Mojave. 

Now, when I do CMD+R at startup, it only offers for me to reinstall LION.

Does anyone have any advice? 

I really would like to be on Mojave because it is much better and faster….., right?

I'm planning on putting in a new battery, a Samsung 860 EVO SSD, and 2x8GB of Corsair RAM after that.

Does anyone have any good advice they can offer in this situation? Thank