Bring a large pot of salted water to a boil and cook the egg noodles per the package instructions, then drain.
While the noodles cook, melt the butter in a large skillet over medium-high heat. Toss in the onion, garlic, peas, and carrots, and cook for about 4-5 minutes, stirring now and then, until the onion softens and turns translucent.
Sprinkle the flour over the vegetables and stir well until it's fully absorbed and no dry patches remain.
Pour in the chicken broth and heavy cream, then whisk in the ranch seasoning. Let the mixture come to a gentle simmer and cook for 3-4 minutes, until it thickens slightly.
Add the rotisserie chicken and drained noodles to the skillet, tossing everything together until heated through. Taste and adjust with salt and pepper before serving.
Notes
Cook the flour into the vegetables fully before adding liquid to avoid a lumpy sauce.
Slightly undercook the noodles, since they finish softening once tossed in the hot sauce.
Rotisserie chicken adds built-in seasoning and keeps prep to about 25 minutes.
Swap heavy cream for milk for a lighter sauce, or add cheese for a richer one.
Leftovers keep 3 days in the fridge; add a splash of milk or broth when reheating.
Not recommended for freezing, since the cream sauce can separate once thawed.