Preheat oven to 375°F. Spray a 13 x 9 inch baking dish with cooking spray, set aside.
Cook spaghetti in salted water per the package instructions until al dente. Drain well.
Meanwhile, in a Dutch oven or large pot (It will double as the mixing bowl) cook bacon over medium-high heat until crisp. Remove to paper towels to drain and chop. Reserve 3 tablespoons drippings in pot.
To the drippings add green onions and minced garlic. Cook for 1-2 minutes then add flour, mixing until liquid is absorbed. To the pot whisk in milk, chicken stock, salt and black pepper, Mix well, bring to a gentle boil then immediately lower the heat. Let simmer for 5 minutes until thickened.
To the pot add Ranch dressing mix, Rotel tomatoes and cream cheese. Mix until cream cheese melts into the sauce. Add 2 cups cheddar cheese, mix stirring until melted.
Remove from the heat, add drained spaghetti, chopped chicken and cooked bacon. (Reserve some bacon for garnishing.)
Mix with a large spoon until fully combined then pour into the prepared baking dish. Sprinkle with reserved shredded cheese. (If pot used is oven safe, there's no need to transfer to a dish.)
Bake for 30-40 minutes or until bubbly around the edges.
Let stand 5 minutes, then serve garnished with green onion and reserved bacon.