。
She spread a thick layer of honey on her toast.
Literal
She [topic-は] toast [genitive-の] top [at-に] honey [object-を] thickly spread.
厚く is the adverbial form of the い-adjective 厚い ('thick'), formed by the productive ~い→~く swap that turns adjectives into adverbs (早い→早く, おいしい→おいしく). 塗る is the standard verb for spreading or applying anything in a layer — paint, butter, ointment, sunscreen — with the surface marked by に and the substance by を. ハチミツ written in katakana is unmarked and very common for 蜂蜜 in everyday text; 蜂 ('bee') and 蜜 ('nectar/honey') are not part of the standard schoolchild kanji set, so kana spelling is the default outside of food packaging. トースト itself is a loanword from English.