Preheat the oven to 350ºF. Grease a 9x13-inch casserole dish.
In a medium bowl, combine chicken with ½ teaspoon salt, ½ teaspoon pepper, and 2 tablespoons ranch dressing mix.
In a large skillet over medium high heat, melt 2 tablespoons butter and 1 tablespoon olive oil.
Add a single layer of chicken. Cook until golden on each side, about 4 to 5 minutes per side. Remove with a slotted spoon.
Repeat with the remaining chicken, using 2 more tablespoons butter and the remaining 1 tablespoon olive oil.
Reduce heat to medium. Saute onion in pan drippings until soft. Add garlic and saute for 1 more minute.
To the same pan (with the onion and garlic), add 3 tablespoons butter. When butter is melted, add flour, Italian seasoning, basil, remaining 1 teaspoon salt, and remaining 1 tablespoon ranch dressing mix. Stir.
Using a whisk, gradually add chicken broth (¼ to ½ cup at a time), stirring to combine well with the flour mixture.
Add heavy cream. Continue to stir, heating until bubbly.
Add parmesan and stir until cheese is melted. Remove from heat.
To the sauce, add 8 slices of chopped bacon, sun-dried tomatoes, chicken, and pasta.
Combine the pasta with the sauce.
Pour the pasta mixture into the greased dish.
Sprinkle with the remaining 4 pieces of chopped bacon. Top with Italian cheese. Sprinkle with dried or fresh chopped parsley, if desired.
Bake uncovered for 20 minutes or until cheese is melted and the edges are bubbly.