Make a deep vertical incision inside each chicken breast so that it can be stuffed and season the meat with salt and pepper.
Cut the chorizo and cheddar cheese into several equal circles (about 6 chorizo circles and 6 cheddar cheese circles for each chicken breast). Alternate the chorizo circles and cheddar cheese circles on a wooden skewer, push them into the incisions you made in the chicken breasts, and remove the wooden skewers.
Wash the potatoes and cut them into several thin scalloped potato slices. Arrange the potato slices in a rectangle on a piece of plastic wrap so that the slices overlap – the size of the rectangle should be slightly larger than the chicken breast. Repeat this a second time for a total of 2 scalloped potato rectangles.
Place a stuffed chicken breast on each scalloped potato rectangle and wrap the plastic wrap around each chicken breast so that the potato slices mold to the chicken breast. Then carefully remove the plastic wrap.
Cut the mixed tomatoes in half, cut the red onions into wedges, and add them to a casserole dish. Season the dish with some olive oil, salt, and pepper to taste and mix to combine. Place the stuffed chicken breasts on top of the vegetables, transfer the casserole dish to a preheated oven, and cook at 350°F for 30 minutes. Serve the scalloped potato chicken breasts and roasted vegetables with pesto.
