Heat a large sauce pot or fry pan over medium high heat. Add the butter and melt. Add the onions and cook for a couple of minutes.
Add the orzo and cook with the onions & garlic for 3 to 4 minutes, stirring frequently so the orzo doesn't burn. If the pan seems too hot, turn down the heat to medium.
While this is going on, I like to heat up the chicken stock so I'm not adding cold stock to the hot pan with the orzo. No reason to slow down the cooking process by cooling down a hot pan.
Add the chicken stock to the pan and bring it to a boil. Lower heat and simmer uncovered for about 10 minutes while the orzo absorbs the stock. Be sure to stir every so often.
Add the grape tomatoes, chicken and parsley.
Season with salt and pepper and then cover and cook for a few more minutes until all the stock has been absorbed into the orzo. Be sure to stir so the orzo doesn't stick to the pan.
I like stir in the grated Parmesan cheese at the end just before serving. You can add a sprig of fresh parsley to the plate to make it look nice. I always forget.