Preheat the oven to 400° F. Using a sharp knife, carefully poke holes around the spaghetti squash and microwave it for 5 minutes. Let it cool for 5 minutes before carefully slicing it open lengthwise (it will be very hot!). Scoop out the seeds and dispose.
Place the spaghetti squash flesh side down on a rimmed baking sheet with 1/4 cup of water and bake for 25-30 minutes. Remove the spaghetti squash from the oven, flip it over and let it cool for a few minutes before using a fork to pull the strands from the squash boats. Transfer the spaghetti squash strands to a mixing bowl and set aside.
While the squash is baking, heat a large non-stick skillet on medium-high heat with 1 tbsp of olive oil. Add the onion and ground chicken to the skillet. Cook until the chicken has browned and is fully cooked through, crumbling it as you go, drain the excess fat. Sprinkle the taco seasoning over the chicken.
Add the diced tomatoes to the skillet with the chicken, reduce the heat to medium-low and mix until fully combined and tomatoes are warm. Transfer the ground chicken and tomatoes to the large mixing bowl with the squash strands. Toss until fully combined and divide the mixture between both squash boats. Divide the shredded cheese between each squash boat and bake for 5-10 minutes, or until the cheese is melted.
Add your favorite toppings and serve! I topped mine with corn, pickled onions, cilantro, avocado, and a squeeze of lime juice.