Cut the cauliflower into very small, evenly sized florets, aiming for pieces that are similar in size to a traditional stuffing crumb. Finely dice the onion and celery, mince the garlic, and chop the mushrooms if using.

Melt the butter in a large skillet or Dutch oven over medium heat. Add the onion and celery, then cook for about 5 minutes, stirring occasionally, until they begin to soften.

Add the garlic and optional mushrooms to the skillet. Cook for another 3 to 5 minutes, stirring as needed, until the mushrooms release their moisture and the garlic smells fragrant.

Stir in the cauliflower, dried sage, thyme, rosemary or poultry seasoning, salt, and black pepper. Mix thoroughly so the butter and herbs coat the vegetables evenly.

Cover the skillet and cook for 10 to 12 minutes, stirring occasionally, until the cauliflower is tender but not mushy. If excess moisture remains, uncover the pan for the final few minutes so the liquid can evaporate and the edges can brown slightly.

Taste the stuffing and adjust the salt and pepper. Fold in the chopped fresh parsley just before serving, then bring the skillet to the table while the stuffing is warm.