Toss frozen fries with rosemary. Bake on parchment paper-lined baking sheet according to package directions.
Meanwhile, in a medium saucepan, whisk 1 tbsp of the broth with cornstarch until smooth. Whisk in remaining broth. Bring to boil over medium-high heat then immediately reduce to a simmer. Cook, whisking frequently, until gravy thickens to desired consistency, 1 to 2 min. Remove from heat and set aside.
Divide hot fries among 4 plates, and top each serving with chicken, cheese and hot gravy. Serve immediately.
To make the poutine less of a food truck-style indulgence, omit the second package of sweet potato fries and simply divide one package of fries among four plates for a smaller serving.