If "toasty" is a noun (it's not, except in the context of this game), then the correct grammar would be "make a toasty", if "toasty" is an adjective (it's not, you are transforming a card into a toasty that's in your deck), then the correct grammar would be "toast". You stupid piece of shit.
Viewing post in Shroom and Gloom: Jam Version comments
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.