Boil water and cook the pasta for 2 minutes less than package instructions. Save ½ cup of the cooking water. Drain the pasta and set aside. Do the remaining steps while the pasta is cooking.
Heat a large pan over med-high heat and drizzle in 1-2 T olive oil. When the pan is hot, add the diced chicken and season with some salt and pepper. Sauté the chicken until browned and cooked through. Set aside.
In a large bowl, stir together the pesto and ricotta cheese. (You can also do this in the large pot you cooked the pasta in to save a dish.)
Add the cooked pasta, mozzarella cheese, tomatoes, and chicken in with the pesto mixture and toss to coat everything evenly.
If the pasta looks dry, add a splash (not all) of the reserved pasta water as needed to get the mixture to a more saucy consistency. (It should not be swimming in the sauce! A splash or two of water is plenty!)
Pour the mixture into a greased casserole dish and sprinkle with parmesan cheese.
Bake at 350°F 30-35 minutes, until hot and bubbly. Don't over-brown.
Notes
Don't over-brown. Gluten-free noodles get way to crunchy and become hard to eat if you do! Cover with foil if the top is getting too brown before it's hot in the center.
If you want to make this ahead, keep it refrigerated. Make it up to one day in advance.
When you're ready to bake, remove the dish from the refrigerator while you heat up the oven. It will need more time to cook if the casserole is still cold when it goes into the oven.
While it's best fresh, the leftovers do heat up ok.