You are wrong, it's meant make *this mushroom* toasty, so "make toasty"
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https://dictionary.cambridge.org/dictionary/english/toasty#google_vignette
Toasty IS an adjective, meaning: "Comfortably and pleasantly warm". You could say "toast mushroom", and that would indeed be correct. But by that process, the mushroom is literally made toasty. So to say "make toasty", in reference to the mushroom, is absolutely valid.