。
She pounded the hamburger meat flat.
Literal
She [topic-は] hamburger [of-の] meat [object-を] pounded [and-て] flat made.
ハンバーグ in Japan refers specifically to the dish ハンバーグステーキ ('Hamburg steak') — a cooked patty of seasoned ground meat served on a plate with rice, sauce, and vegetables. It's a foundational dish in Japanese family cooking and 洋食 (Western-style restaurants), distinct from ハンバーガー ('hamburger,' a sandwich). 平たい ('flat') becomes 平たく as the adverbial form, attaching to する ('do, make') to give 'make X flat' — a productive 'i-adjective adverbial + する' pattern: 美しくする ('make beautiful'), 大きくする ('make big'), 平たくする ('flatten').