Cook and crumble the bacon. Set aside for later. Cook the pasta according to the box directions. When the pasta is done, drain it in a colander (do not rinse it), and reserve it for later.
In a large skillet pan, over medium heat, add the butter and diced onion. Cook until the onion is soft and translucent and the butter has melted; about 5 minutes.
Add the flour, ranch seasoning mix, salt and pepper into the skillet pan. Use a silicone coated whisk to whisk until it forms a smooth and a thick paste.
Turn the heat up to medium-high and let the mixture simmer, while stirring frequently, until it starts to lightly boil around the edges of the pan and it has thickened.