Slice the ends off of your onions and peel. Place an onion flat on your cutting board on one of the flat sides where the ends were cut off. Cut each onion all the way through in a star pattern to create petals of onion. You simply need to make sure they seem adequately sized for sandwiches and then pack the onion petals into a quart-sized mason jar. One onion per jar.

Roughly chop garlic cloves and add half into each jar with the onions. Add half of your pimento peppers to each jar. Try to pack them down a little. 

Add vinegar, water, sugar, salt, and msg to a small saucepan over medium-high heat. 

Bring the vinegar mixture to a boil. As soon as it starts boiling, stir the mixture, remove it from the heat, and pour it into the jar over the onions, peppers, and garlic. This is obviously a hot pot pouring hot liquid so be careful. I typically do the pouring process in the sink so any spilled near-boiling liquid does not hurt anyone. 

Close the jar lid and let it rest on your counter until cool. 

Add to the fridge and enjoy some great pickled onion slices on your sandwiches over the next few weeks.