PUZZLE POEM

I am a room but I have no floor 
No windows either and definitely no door 
I keep you warm during winter time 
So what is the answer to my rhyme ?

I end up in a dish or perhaps on a plate 
Served with eggs I taste just great 
I grow wild in a field with others like me 
You can pick me up for I am free 

My shape is rather weird and strange 
Been like this for centuries I won't change 
My first and last letter are both the same 
For I am a mushroom so ending this silly game