Preheat your oven to 375°F (190°C). Lightly oil a 9 x 13 a9 x 13. baking dish, 1 inch. While the oven is preheating, fry the bacon in a skillet over medium-high heat until crispy, about 8 to 10 minutes. When the bacon cools, move it to a dish covered with paper towels and slice it into bite-sized pieces. Season the chicken breasts on both sides with salt and pepper. To prepare the pasta, bring a pot of salted water to a boil.
Stir the softened cream cheese, sour cream, ranch seasoning mix, salt, pepper, and garlic powder in a big bowl until completely combined and smooth. Stir the uncooked chicken breasts into the cream cheese mixture until all the pieces are uniformly coated. Spread the mixture evenly in your baking dish after transferring it there. The chicken should be partially cooked and beginning to firm up after 20 minutes in the oven.
To keep the shell pasta from being mushy when baked with the creamy sauce, cook it in the boiling salted water until it is just al dente, or slightly firm to the bite, while the chicken bakes. Don't rinse the pasta; instead, drain it in a strainer. When the chicken is cool enough to handle after 20 minutes of baking, take it out of the oven (keeping the oven at 375°F) and shred it immediately in the baking dish with two forks.
Stir thoroughly to combine the shredded chicken and cream sauce. Add the chopped bacon from Step 1, the cooked pasta from Step 3, and half of the combined cheeses (1 cup total: roughly 1/2 cup mozzarella and 1/2 cup sharp cheddar). Gently stir the spaghetti and bacon into the creamy chicken mixture until they are evenly distributed.
Over the top of the dish, equally distribute the remaining cheese (1/2 cup strong cheddar and 1/2 cup mozzarella). Bake for five to ten minutes, or until the cheese starts to melt, at 375°F (190°C). To achieve a golden, somewhat crispy cheese top, turn your oven on to the broil setting on high and broil for precisely three minutes.
After taking the casserole out of the oven, give it two to three minutes to gradually solidify. Serve straight from the baking dish after generously garnishing with the chopped green onions.