I worked for over 5 years at the Cheesecake factory and can definitely tell you it is shallots, not red onion in the recipe. I can also tell you a bit of heavy cream is added to the sauce and there is no parsley/basil/or cilantro. A good madiera sauce definitely depends on the wine. I have made it a few times at home and can get a pretty good copy but its definitely not exact. I don't have any idea of measurements but this is what I do

first I saute the chicken and cook the shallots and mushrooms
for the sauce I use
about
2 tablespoons of butter
2 tablespoons of flour
about a cup and a half of madiera wine
splash of heavy cream
add shallots and mushrooms and simmer for a few minutes
then add chicken and cooked pasta and voila