Preheat your oven to 375°F and lightly grease a 9x13 inch baking dish. While the oven heats, cook the bacon in a skillet over medium-high heat until crispy, about 8-10 minutes, then transfer to a paper towel-lined plate and chop into bite-sized pieces once cooled. Start boiling a pot of salted water for the pasta—this can happen simultaneously with bacon cooking. Season the chicken breasts with the salt and pepper on both sides, and have all other ingredients measured and ready before you begin mixing.

In a large bowl, combine the softened cream cheese, sour cream, ranch seasoning mix, salt, pepper, and garlic powder, stirring until smooth and well blended. Add the raw chicken breasts to the cream cheese mixture and stir to coat all pieces evenly. Transfer the mixture to your prepared baking dish, spreading it into an even layer. Bake for 20 minutes until the chicken is partially cooked and starting to firm up.

While the chicken bakes, cook the shell pasta in the boiling salted water until just al dente (still slightly firm to the bite), which prevents it from becoming mushy when baked with the creamy sauce. Drain the pasta in a colander but don't rinse it—the starch helps it bind with the sauce. Once the chicken has baked for 20 minutes and is cool enough to handle, remove it from the oven (keeping the oven at 375°F) and shred the chicken directly in the baking dish using two forks.

Give the shredded chicken and cream sauce a good stir to mix thoroughly. Add the cooked pasta from Step 3, the chopped bacon from Step 1, and half of the combined cheeses (1 cup total: about 1/2 cup sharp cheddar and 1/2 cup mozzarella). Stir everything together gently until the pasta and bacon are evenly distributed throughout the creamy chicken mixture. I like to do this final stir carefully so the pasta doesn't break apart.

Sprinkle the remaining cheese (1/2 cup sharp cheddar and 1/2 cup mozzarella) evenly over the top of the casserole. Bake at 375°F for 5-10 minutes until the cheese begins to melt. Then switch your oven to the broil setting on high and broil for exactly 3 minutes to create a golden, slightly crispy cheese top—watch carefully to prevent burning.

Remove the casserole from the oven and let it rest for 2-3 minutes to set slightly. This resting time helps the casserole hold together better when serving. Garnish generously with the sliced green onions and serve directly from the baking dish.