Here's a mushroom gravy I created recently. Best on spaetzle (German egg noodles--you can buy them at most grocery stores, or, if you have a spaetzle-maker, make them very easily yourself). The gravy would probably work on potatoes too. Can serve with bratwurst, and either sauerkraut or rotkohl (I'd recommend the rotkohl). This is probably more authentically German than many of your options at a German restaurant. And it tastes better.
Chop up and saute 1 large onion (I use an electrical chopper that basically beats the onion into a pulp).
Saute 1.5 lbs sliced mushrooms.
Dissolve one beef bouillon cube in 1.5 cups heated water, and then add two tablespoons cornstarch to the broth. Mix, get all the lumps out, and then add the onions and the mushrooms to the mix. Add just a little milk or sour cream if it needs a little thinning, and add salt and pepper to taste. It should be very thick with mushrooms.
The Germans aren't the greatest cooks in the world, but they do know how to bake, they know how to properly cook cabbage (by turning it into sauerkraut or rotkohl) and they know how to prepare mushrooms. I wasn't a mushroom fan until I tasted them like this, done right. Now I can't get enough of them. I've even converted April.
Next up: Ruth's Rotkohl, stolen from a college friend who also served in Germany.