Multi-layer Parts of Speech
Not only single words, phrases and clauses can also work as nouns, verbs, adverbs, adjectives etc.
This makes parts of speech multi-layered.
What part of a sentence works as what part of speech will depend on whether we analyse the sentence at individual word level or at phrase or clause level.
Add a small spoonful of common salt, then turn up the heat and move away from it.
(add - verb; a - determiner; small - adjective; spoonful - noun; of - preposition; common - adjective; salt - noun)
(add - verb; a small spoonful - determiner; of - preposition; common salt - noun)
(add - verb; a small spoonful of common salt - noun phrase)
As far as the second part of the sentence is concerned:
(then - conjunction; turn up the heat - verb phrase; and - conjunction; move away from it - verb phrase)
There is no single correct way of looking at this. All these perspectives are correct. You should choose the one that makes more sense.