Pour oil into a pan and heat it up while slicing the onions.
Add the onions with salt into the hot oil and saute until it get a glassy / translucent look for about 3 - 5 minutes.
Lower the heat to medium - low and fry the onions while stirring continously for about 10 minutes.
When the onions start to caramelize add the garlic, black pepper and mushrooms.
Increase the heat to medium and saute the mushrooms with the onion until the mushrooms get a dark brown color for about 5 - 10 minutes.
While the filling is being prepared get the meat ready by removing the silver skin from the pork tenderloin. It's a thin layer of connective tissue that is said to be very chewy. Some can live with it. Especially those with blunt knifes.
Open up the meat. There are two methods. One is to cut it almost all the way through into halves and use a meat mallet to pound it wider. The other method is using only our sharp knife to cut various butterfly cuts into the meat until it opens up fully. Cut 1 about 1/4th of the bottom:
Cut 2. After opening up the tenderloin cut the thick part into two again essentially butterfly cutting it again.
Cut 3. Cut the last thick part at the middle of its height too.It's opened up. Sprinkle some salt on it if desired.
Place the handful of spinach on the opened up meat.
Pour as much of filling as possible on top of the spinach.
Try to roll up the meat and take out some filling if necessary.
Use the turkey lacers or toothpicks to secure the meat together.
Pour the remaining filling with the oil into a baking tray.
Place the stuffed pork tenderloin in and brush it with a mixture of oil and paprika.
Add vegetables if desired.
Put it into a 390°F / 200°C oven until the inner temperature of the meat reaches 145°F / 63°C which should be done in about a half an hour. Some like to give it a bit more time so it comes out with a bit more tan. After about an hour it should have a golden brown color and that's the time when it needs to be taken out to save from total blackness.
Slice it up with a sharp knife and serve.
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