Decide what your gravy is missing: dark color, savory depth, or thick body (or multiple).
Choose the closest substitute: use roux for richness/thickening, cornstarch slurry for speed, bouillon for savoriness, or onion soup mix for a quick all-in-one boost.
Add gradually (start small), whisk well, and simmer briefly so the gravy’s texture stabilizes.
Adjust in the right direction: add a splash of stock for thinness, more thickener if it’s watery, or an extra pinch of seasoning if it tastes flat.
Stop when it coats a spoon; gravy thickens slightly as it cools.