Failed pan sauce didn't thicken and tasted far too much of wine
The biggest reason your sauce didn't thicken is that you didn't have much of anything at all in the pan that will gelatinize and help trap the water molecules present in the sauce.  Starches (flour, cornstarch) will provide some of this, as will a liquid like stock that contains some dissolved collagens.  But wine and water by themselves will have very little thickening power.  At minimum, you'll want to add a teaspoon or two of cornstarch to your wine/water mix and shake in a covered vessel - this is called a slurry and will thicken fairly quickly, but you need to be careful not to overcook it.

For a more traditional (and in my opinion, much tastier) pan sauce, try the following steps:

First, make sure that you pour off any excess oil - you want no more than about a tablespoon or so left in the pan, less if you're adding butter.

Second, heat the pan by itself for a few moments, which will help dissolve the fond (little flavorful brown bits left in the pan) during the next steps.  If you want to add your onions, this would be the time to do so.

At this point, if you want a really thick sauce, add your butter, and sprinkle in a very small amount (no more than the amount of butter you've added) of regular flour.  Whisk this well for about a minute until it seems to thin slightly.  What you're doing here is creating a quick roux that will help thicken the final mixture.  You can skip this step if you like.

Next, carefully add your wine and wait a moment to help burn off some of the alcohol flavor.  Whisk or stir with a fork for few seconds to help dissolve the flavorful fond.  Then, add stock - this is very advisable especially if you didn't create a roux.  Stock both has more flavor than water and (especially with meat stocks) contains collagen, producing a more velvety-tasting final sauce.  Store-bought stocks are fine, and are a great pantry item to keep around; you don't have to make your own.

At this point, stir in your other flavorings (herbs, etc.) and reduce the sauce over medium-low heat until you're happy with the texture.  A good way of checking is to dip in a spoon, flip it over, and run your finger across the back.  If the sauce doesn't fill in the track left by your finger, it's good to go.