Preheat oven to 425 degrees. Place the sheet pan in the oven to heat at the same time. (Note 1)
Cut the chicken into 6 4-ounce pieces. Set aside. On a clean work surface, cut potatoes in half, leaving smallest ones whole.
Add the potatoes to a zipper-top bag or mixing bowl. Add the oil and toss to coat. Add ½ of the ranch dressing mix and toss to coat. Transfer the potatoes to the hot sheet pan. Drizzle the chicken breasts with oil and sprinkle with the remaining ½ of the ranch dressing mix. Turn to coat.
Place the chicken in the bag (or bowl) and add the remaining dressing mix. Toss to coat. Arrange the chicken on the pan in between and around the potatoes. Bake for 18-25 minutes or until the chicken reaches an internal temperature of 165 degrees (using an instant-read thermometer).
Meanwhile, add the bacon to a skillet and cook until brown and crisp. Drain on a paper towel-lined plate. Chop and set aside. Grate the cheese.
Remove the pan from the oven and top the chicken and potatoes with the cheese and bacon. Bake for another 3-5 minutes or until the cheese is melted. Garnish with chopped chives or scallions and serve.
Preheated the sheet pan helps to prevent the food from sticking. It also helps to jump start the cooking process (indicated by the sizzle when the food hits the pan).
Use freshly grated cheese for optimal melting. Packaged pre-shredded cheese has additives to prevent clumping which inhibits its meltability.
Use a half sheet pan so the chicken and potatoes are not overcrowded. An overcrowded pan will cause the ingredients to steam, not roast.
Always cook chicken by temperature, not time. Cook chicken until the internal temperature reaches 165 degrees. An instant-read thermometer is the most accurate way to tell if the chicken is cooked to a safe temperature.