Using a crockpot, cook chicken and chicken broth on high for 3 hours. Add enough chicken broth to cover over half of the chicken (no need to completely submerse chicken).
Remove chicken breasts and shred them using a fork, then bring it back to cook on low in crockpot for 30 minutes.
Cook spaghetti pasta in a pot for 8-9 minutes, or two minutes less than package recommendation. Drain and set aside.
In a pot, sauté minced garlic in olive oil for 30 seconds. Do not over-brown or burn garlic.
Add diced tomatoes, mascarpone cheese, 3/4 cup of cooked chicken broth from the crockpot (without the chicken), basil, oregano, and marjoram.
Bring to boil, then cook on medium low heat for 10-15 minutes. Add 1/3 cup (packed) of shredded mozzarella cheese. Liquid should thicken slightly. If you like the liquid thicker, add another 1/4 cup (or more) of shredded mozzarella cheese. Keep in mind that liquid will thicken more after adding the more items. Add salt to taste.
Remove from heat and add shredded chicken (drained) and cooked spaghetti into the sauce. Mix well. Then pour everything into a 9"x13" casserole dish.
Sprinkle the rest of the shredded mozzarella onto the top of the casserole dish.
Broil casserole dish in oven for 3-4 minutes until the cheese on the top of the casserole is browned.
Remove from oven and cool for 10 minutes. Serve and enjoy!