Bring a large pot of salted water to a boil. Add the pasta and cook until al dente, about 8 minutes. Drain and set aside.
Meanwhile, melt the butter in a large skillet or saucepan over medium heat. Add the onion and garlic. Season with the pepper and a good pinch of salt and cook until the onion and garlic are softened, 3 to 4 minutes.
Sprinkle the flour over the onion and garlic and stir to make a roux. Let cook 2 to 3 minutes before slowly stirring in the milk and then the heavy cream. Bring to a gentle boil, allowing the mixture to thicken slightly, 2 to 3 minutes.
Reduce the heat to low and stir in the cheddar and pepper jack a handful at a time, allowing each addition to melt before adding the next. Stir in the chicken, hot sauce, and ranch dressing and cook until the chicken is heated through, 1 to 2 minutes. Taste and adjust seasonings as needed. Add the cooked pasta to the sauce and toss well to combine.
Serve topped with the blue cheese, celery leaves, and a drizzle of hot sauce, if you like.
