I've had Jaegerschnitzel several different ways, so I think you have flexibility in the sauce. Some use cream, some don't. I think I prefer it without.
I would just start by sweating some onions in butter, add mushoorms and cook down, sprinkle lightly with flour to make a roux, then hit it with some wine (white or red would work I think, depends on the meat you're using). Let the wine cook down, then add some stock (chicken or beef, but veal would be best). Reduce to proper thickness. Season. It's a basic pan sauce.
If you want to use cream, add it towards the end. I would probably use white wine and some veal demiglace instead of veal stock since I have demi in the fridge. If I was making it with beef, I'd use red wine. Pork, I'd use white wine. Probably white with veal too, but a light red would work.
I don't think I'd use sour cream. If you did that, I'd call it a stroganoff. Especially if you serve it with noodles instead of spaetzle. I'd avoid cream for the same reason. But it's easy enough to make the sauce and then split it towards the end and put cream in one half. Try it and see which you prefer.