Cut the spaghetti squash in half and scoop out all the seeds and stringy pulp. (Gross!)
Place halves in microwave, scooped side up, and microwave for about 30-40 minutes. You can butter the squash a bit before microwaving, if you like.
Meanwhile, break an egg into a shall bowl and whisk it.
Pour breadcrumbs onto a plate.
Dip chicken breasts in egg, then coat with breadcrumbs on the plate. Place chicken in an oven-safe baking dish and cook for 45 to 60 minutes (depending on the size of the breasts) at 350 degrees (F).
When chicken's juices run clear, pour the can of crushed tomatoes over the top and place mozzarella cheese on top. Return pan to oven and cook for another 15-20 minutes, until cheese is melted and slightly browned.
While the chicken is baking, the spaghetti squash will finish cooking in the microwave. Allow it to cool, then use a fork to pull it out of the squash shells. When the chicken is finished cooking, divide spaghetti squash into bowls and place chicken breast on top, along with some of the tomato sauce. Sprinkle parmesan cheese on top and enjoy!