Choose your substitute based on how you’re cooking: long-simmer dishes need sturdy greens (mustard, kale, turnip).
Match texture by prep: de-stem and chop sturdy greens smaller so they soften like collards.
Adjust time: greens that cook faster (chard, turnip) need less simmering; spinach must be added at the end.
Balance flavor: if your substitute is more bitter than collards, simmer with a small splash of vinegar or lemon.
Taste and tune: add salt and seasoning after the greens soften, then simmer 2–3 minutes more to blend flavors.