Declare a global script level variable, then make the mock operate on that variable. Parameters will be automatically injected into the scope of the mock function

funciton My-RealFunction {
	param($Stuff)
	...do something with stuff...
}

[stirng[]]$script:invokations = @()

Mock My-RealFunction {
	$script:invokations += $Stuff #Stuff is injected automatically
}

... assertions based on $script:invokations ...