Is it "stir fry", "stir-fry" or "stir-fried"

Solution 1:

The verb forms stir-fry with a hyphen and stir fry without one are interchangeable. The choice is between you and your style guide or editor. Pick one and stick with it. For example, Merriam-Webster, the dictionary that The Chicago Manual of Style recommends, uses the hyphen—as does the OED.

Using the hyphen in the verb form is useful because its derivatives—the adjective and noun forms—will also always use a hyphen.

I stir-fry very day. (intransitive verb)
I stir-fry vegetables. (transitive verb)
I like stir-fried vegetables. (attributive adjective)
My vegetables are stir-fried. (predicate adjective)
I like stir-fry. (noun)
I like the stir-fry technique. (attributive noun or attributive verb phrase—functions as adjective).

Compare that to the open, un-hyphenated verb form, where—depending on the style guide—the adjective and noun derivatives might be open or hyphenated. You might find a rule like this:

I like stir-fried vegetables. (hyphenated in the attributive position)
My vegetables are stir fried. (open in the predicate position)

Things could get messy.

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Are stir-fry and stir-fried interchangeable when used as adjectives? Not really. Though Collins and perhaps others classify stir-fry as an adjective, other dictionaries see stir-fry as a noun or verb form temporarily repurposed as an adjective.* Depending on the noun being modified and the position in the sentence, only certain combinations work.

I like stir-fried vegetables.
? I like stir-fry vegetables. (odd)

I'll order the stir-fry plate.
*I'll order the stir-fried plate. (grammatical but incorrect meaning)

My vegetables are stir-fried.
? My vegetables are stir-fry. (odd; in any case, stir-fry becomes a predicate nominative here)

I like the stir-fry technique.
*I like the stir-fried technique. (grammatical but incorrect meaning)

(Thank you to @JasonBassford for the discussion in the question comments.)

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*The OED classifies stir-fry as a verb and a noun, but not an adjective. Used in front of a noun, it's a "verbal phrase used attributively." Stir-fried is classified as an adjective.

stir-fry, v.

1. transitive. Chiefly in Chinese cookery: to fry (meat, vegetables, etc.) rapidly on a high heat, while stirring and tossing them in the pan. Also intransitive.
_{1969   Guardian 16 July 16/4: Stir-fry the vegetables.}

 2. The verbal phrase used attributively.
_{1981   J. MANN Funeral Sites xii. 77: Ian..had spent some time making a Chinese stir-fry dinner.}

DERIVATIVES

stir-fried adj.
_{1981   Sunday Tel. 6 Dec. 13/5: Stir-fried shredded beef with bean sprouts and celery.}

stir-frying  n.
_{1980   Redbook Oct. 136/2: To save time and money in the kitchen, Denise does a lot of stir-frying and uses a slow cooker.}

And:

stir-fry, n.
_{Etymology: < STIR-FRY v.}

Food or a dish prepared by stir-frying.
_{2013   Self Apr. 56 (caption): Head off soreness by chowing on stir-fry with a few teaspoons of ginger.}

_{Definition and examples source: Oxford English Dictionary (login required)}