With that colouration I'd agree....however that cap has that very 'gritty' look of an Armillaria! Any chance you would be able to get another image as it matures please to help confirm identification. Great image
No respect!!! Even though I'm feeling it is an Armillaria sp. as I can't be absolutely sure will just leave the identification at 'Agaric' a gilled fungus, sorry. Keep an eye out for it in the future though and if you see it try and take a photo of it at a more mature stage. Maybe if it does come up again warn people not to cover it up!!
I've just realised you said you found these fungi growning in your mulch heap and not in the soil. So I'm going to agree with you and confirm your id as G. dipelis. Well done.