Q: This is a hamburg steak I cooked yesterday. この表現は自然ですか?
A: × This is a hamburg steak I cooked yesterday.
✓ I cooked this hamburger steak yesterday.

Q: "I made hamburg stake last night. It was delicious."

Hello! Do you think the sentence above sounds natural? Do I have to say "a hamburg steak"? "a" is necessary?
A: Hmmm…I think you mean either hamburger or steak. Never hamburg, and a hamburger steak isn’t a thing.
Q: I ate a Japanese hamburg in a Japanese restaurant. It was so nice for me, it was so juicy and simple seasoning. この表現は自然ですか?
A: Does your Japanese hamburger ハンバーグ (hanbagu) look like this? if yes, it's called a Japanese hamburger steak, the hamburger buns are missing 😅

Simple seasoning would imply just salt and pepper. I recalled it was drenched in a sweet umami gravy.
Q: The hamburg steak which I made today was collapsed.Maybe I should've kneaded it more before grilled. この表現は自然ですか?
A: The hamburger steak that I made today had collapsed. Maybe I should've kneaded it more before I put it on the grill.
Q: I had eaten a hamburg in the McDonalds when I come back to my home from the gym. この表現は自然ですか?
A: It sounds good :) but maybe try
"I had a hamburger from McDonalds when I got home from the gym"
To make it seem a bit more natural