Proably the clearest way to approach this would be to build a list of slices to eat, then to process it, avoiding altering the original enumeration in-loop. I've never been a fan of using indexed loops for this, as it can be error-prone.
List slicesToEat=new List();
foreach(var Slice in Pizza)
{
if(Slice.Flavor == "Sausage")
{
slicesToEat.Add(Slice);
}
}
foreach(var slice in slicesToEat)
{
Me.Eat(slice);
}