Beef with ratatouille

I'm not sure this dish was successful because the beef was not tender enough even though I marinated them in light vinegar beforehand.  It was probably better with chicken.

「牛肉のラタトウイユ煮」軽くお酢でマリネした牛肉を野菜と煮込んだもの。ラタトウイユ風にしたのだが、肉が固くなってしまって失敗気味だった。鶏の方が良かったと思う。