Preheat oven to 400 degrees
Melt 1 tbsp of butter over medium heat in a medium frying pan. Add garlic clove and 2 tbsp of minced red onion. Season with salt and pepper and continue to saute.
Finely chop one beefsteak tomato and add to the frying pan. Stir and cook mixture until liquid has been evaporated and becomes thick. Stir in the Parmesan cheese.
Let cool and transfer mixture to food processor and puree until smooth. Remove from food processor, and season with salt and pepper if necessary. Set aside.
Prepare chicken breasts by setting them on a cutting board. Slide your fingers gently under the skin to loosen and create a space between the skin and the breast to spoon the tomato butter into. Divide the tomato butter between the four chicken breasts, using a small spoon, spoon the mixture under the skin. Press and smooth the top of the chicken breast to help evenly distribute the butter underneath. Season the tops and bottoms of the chicken breasts with salt and pepper. Set aside.
Cut the remaining four tomatoes in half horizontally, and add to a roasting pan. Cut the remaining onion into wedges and add to the pan with the bread croutons and garlic head halves. Mix the ingredients well and drizzle with olive oil. Season well with salt and pepper.
Place the chicken breasts on the bed of vegetables and croutons. Bake for 30-40 minutes or until internal temperature reads 165 degrees Fahrenheit. Remove from oven, cover with foil and let rest for 10 minutes before serving.