For someone in their element

Estar en su salsa

es-TAR en soo SAL-sah — To be in one's element (literally: "to be in one's sauce")

"Con un micrófono delante, Marta está en su salsa."
Put a microphone in front of Marta and she's in her element.

Change the possessive to match the person: estoy en mi salsa, estás en tu salsa. Spanish also has "como pez en el agua" (like a fish in water) for the same idea.

"Honestly? Me, mid-lesson, explaining some bit of grammar nobody else finds exciting."— Graciela