The idea for a pan sauce is that you have already seared your meat in the skillet, leaving behind juices and brown bits. Now, with your skillet under medium heat, add the shallots and olive oil. If lots of pan juices are in the skillet, skip the olive oil. You might even need to drain some of the juices, but be sure to keep the brown bits. You just need enough to cook the shallots without burning them.
Stir and sweat the shallots for 3-4 minutes while they soften
Add the minced garlic and stir until it's fragrant--not more than 30 seconds or so
Add sliced mushrooms and cook until brown and they've lost most of their moisture
De-glaze the pan with the wine. Use a wooden spoon to scrape up the brown bits left over from browning the meat. They will dissolve in the wine and shallot mixture. The pan will sizzle when you add the wine! Fun!
Now add the chicken broth, brandy, and whatever herbs you are using.
For a thicker sauce, make the corn starch slurry and add to the mix. With the corn starch, it should begin to thicken almost at once. Without, it takes a few minutes longer and is generally less thick.
Once thickened (with our without corn starch slurry), remove from heat and stir in butter. If your're adding mustard, this is the time to do it. The sauce thickens further as it cools, which is why the corn starch is only needed if you want a more gravy-like sauce.