...so how the hell do I clean a flask?

Long story short, I have a lovely flask. It is not expensive, maybe stainless steel? But it's important to me because it was a gift from my best friend years ago when I got my first real job. I think the basic flask is stainless steel, and there's overlay on it that could be anything. I found similar flasks online and the only material listed is stainless steel so maybe it's just colored?

ANYWAY, so I recently moved. And I was going through my stuff and thought it would be fun to use this flask, which I had never actually used before. And tried to open the lid, which I could not open. I soaked it in warm water and got it open, only to discover it is crazy moldy and smells of liquor. I suspect it was used by my shitty ex right before we broke up...like 8 years ago (I distinctly remember getting in an argument when she wanted to take it to a party and I said no because I thought she'd lose it...I think she probably took it anyway).

How the hell do I clean this enough to use it? Is that even possible, or should I just disinfect it as much as possible and relegate it to souvenir status? I'm going to visit the friend who gave it to me next month (she lives across the country now) and really wanted to take it as a nod to how much her friendship over the years means to me, but also this is really gross. Help, you clean GTers! I know you must exist!

Okay so that wasn't so short, but please help!