How can I thicken my mushroom sauce? Recently I have been making Crimini mushrooms like this:

Purchase a small container from the store
Quick rinse it at home in water to clean the dirt off
In a small pot, I place a half stick of butter and marscapone and then simmer it
While simmering I add garlic, cumin, salt, pepper, and paprika
And then I add a little bit of soy sauce and warchestershire sauce
Once that is all simmering I add the mushrooms and place the steam lid on the pot (just a glass lid with a small vent in it)
I adjust the temperature so that the sauce rises to just below the lid
I let it cook for about 20 minutes like that and then place the sauce in a bowl

It tastes really good. I like how the mushrooms are very soft and have a nice flavor to them. The sauce tastes good as well. However, the sauce is runny and I cannot figure out how to thicken it. I have tried using various other combinations instead of the butter and marscapone such as half and half, heavy cream, cream cheese, and creme freche but still can't seem to get it thick.

How should I thicken the sauce or what I am doing that is preventing it from thickening?
You could use any number of methods, including:


Starch Thickeners (added as a slurry)

One of the simplest and most straight forward:  dissolve some starch (cornstarch is common in the US; alternates include potato starch, arrowroot, or tapioca) in some water, into a smooth slurry.  

Add the slurry to your simmering sauce base, stirring, and let it cook for a minute or so.  As long as your sauce is at the simmer, it will come to full thickness very quickly.

Add slowly until the desired thickness is reached.
Flour slurry

The technique is similar to using a pure starch:  create a slurry of flour in water, enough water so that it will pour easily.

However, you will want the sauce base at a boil, as flour thickens at higher temperatures.  It will also add more cloudiness to your sauce (probably not a factor with your recipe).

Add slowly until the desired thickness is reached.
With a beurre manee (butter kneaded with flour)

Knead equal parts of room temperature butter and flour together.

Add slowly to your boiling sauce, while whisking, until the desired thickness is reached.
With roux (flour cooked with butter)

Melt butter in a saucepan.  Add an equal volume of flour, whisking it in to prevent lumps.  Continue cooking for a couple minutes.  You can use this right away, or save it in the refrigerator to use later.

Add roux to your boiling sauce base, whisking, until you achieve your desired thickness.


All of these starch based methods will thicken about one to two cups of sauce per tablespoon of starch, depending on how much additional thickness you desire.  The exact ratio of thickener to sauce is up to your taste.

All of the starch based methods should be done at the end of the cooking period, prior to service.  


Reduction

Simply reduce your sauce until it is as thick as you desire.  This may overcook your mushrooms so may not be an ideal method for your recipe.  You might also reduce your sauce to almost the desired thickness, then add the mushrooms.  Your sauce is very, very high in fat (butter and marscapone, so it will not likely break).
More mushrooms (and a blender)

Add even more mushrooms to your recipe.  When it is cooked, remove some mushrooms and reserve them.  Puree the remaining mushrooms and sauce in a blender or with an immersion blender (or even with a food mill).  Add the whole mushrooms back for flavor.  The body of the pureed mushrooms will add thickness to your sauce. 

This method will work even better if you add some potato chunks with your mushrooms.  This gives the benefits of the potato starch, and more body from the potatoes when you puree.