Pork Tenderloin with Mustard Sauce

I got this recipe from my sister and she got it from a Junior League cookbook - I wish I could tell you which one, but I don't remember.  We love this.  It is delicious and a wonderful treat.  Also, if you have any pork left over, you can use it in pork fried rice (which we do all the time).  If you don't want the expense of a 3-pound pork loin - get the little tenderloin.  That's what I used before we had kiddos. 

1/4 cup soy sauce
1/4 cup bourbon
2 Tblsp brown sugar
1 (3-pound) pork tenderloin
1 Tblsp dry mustard
1-1/2 tsp vinegar (I use white for this)
1/2 cup sour cream
1/3 cup mayonnaise (use the real stuff!)
1 Tblsp chopped green onion
salt to taste

To marinate the pork:  Combine the soy sauce, bourbon and brown sugar in a bowl and mix well.  Add the pork.  Marinate in the refrigerator for several hours, turning occasionally.  {I do all of this in a gallon-size Ziploc bag - why dirty up a bowl?}  Drain, reserving the marinade.
To cook the pork: Preheat the oven to 325-degrees, place the pork in a baking pan.  Roast for 45 to 60 minutes or until cooked through, basking frequently with reserved marinade.
For the sauce:  In a bowl, dissolve the dry mustard in the vinegar. Mix in sour cream and mayo.  Stir in the green onion and salt.  Serve with the sliced tenderloin.  (this reminds me a lot of a horseradish kind of sauce - it is delicious with the meat!!)