Heat a medium sized saucepan over medium-high heat. Add the butter and then the chopped shallot. Let this cook until the shallot is translucent, about 2 minutes.
Remove the pan from the heat and deglaze with the red wine. Be careful not to burn yourself. Bring the pan back to the heat, lower it to medium and reduce the wine to an essence.
Just before all the wine has cooked off, add the demi-glace and again lower the heat and simmer the sauce for about 5 minutes.
Add the red currant jelly, fresh tarragon, pepper and keep reducing until the sauce is thick enough to coat a spoon.
If you cook it down too much (something I do a lot if I'm not paying attention) you can add a little extra wine or beef stock.
In restaurants, the chef will often add a pat or two of butter to give the sauce a little more flavor and some extra sheen. It also adds a bunch more calories, so I try not to indulge in this practice unless I'm having guests over.
When the sauce is at your desired thickness, taste and adjust seasonings.
If you want to make the sauce even more elegant, strain it through a fine mesh strainer before serving over your beef, pork, veal or chicken.