Begin by preparing chicken to cook and shred. For Instant pot: add tomato sauce, chicken and seasonings to pot. Select manual mode for 5 minutes and let naturally release for 10 minutes. Remove lid and shred. For slow cooker: add tomato sauce, chicken and seasonings to pot. Cook on low for 6-8 hours. Remove lid and shred.
Meanwhile, heat oven to 375 F and prepare zucchini by slicing in half lengthwise and seeding. Boil a pot of water large enough to place zucchini in. Boil for 3 minutes and remove to casserole dish.
Fill zucchini boats with shredded chicken. Sprinkle cheese over the top. Move to oven and bake for 15 minutes. Remove, let cool slightly and serve.