The challenge provides an encrypted flag.

hlrv{X1g3ett3_2_34sp_bcae}

Searching for the phrase `Omelette du fromage` brought me to an episode of Dexter's Laboratory in which the phrase is repeated over and over. This felt like a clue that the cipher would use repetition. I tried a Vigenere cipher first. Since the first part of the ciphertext is known to say `flag`, I played around with adding key characters to an online Vigenere solver until I got `flag` to be produced in the plaintext output. Thankfully, that was the full key: `carp`, and the flag was revealed.

flag{V1g3ner3_2_34sy_maan}