Preheat the oven to 425.
slice potatoes into 1/3 inch coins (you do not need to peel). Place on a sheetpan lined with foil. Toss with 1 tbsp olive oil, and season with salt and pepper.
Pat the tilapia dry with a paper towel. Place on a baking sheet lined with foil lubricated with cooking spray. Season with salt and pepper. Top each fish filet with 2 tablespoons of parmesan cheese.
Place the fish on the top oven rack and the potatoes on the bottom rack to roast. The fish will be done in about 10-15 minutes (look for the cheese to be melted and browned. The fish should flake easily with a fork). Once you remove the fish from the over, turn the potatoes over and cook for another 5-10 minutes (total cook time for the potatoes should be about 20 minutes)
Top each filet with tomatoes and a tablespoon of pesto, serve.