Yeah I know but after Onslaught was inverted/defeated, how is Red Skull's body alive? His head got splattered like grapefruit.
Don't OFFICIALLY know, but I'll take a crack at it...
In the original story, When Onslaught first appeared, Xavier's body was still being stored inside of Onslaught's shell, in perfect condition, and Thor actually had to rip Xavier out. Nate Grey and Franklin Richards were also both housed inside of his shell as well, in perfect condition.
I don't suppose its out of the question to believe Red Onslaught can do the same with his own body