Place the clean and dry piece of pork sirloin on a cutting board and with a long thin knife make about 5 horizontal cuts through the meat from both sides.
Peal the garlic and slip it into the holes in the meat.
Brown the sirloin with a little oil all around.
Mix the salt and herbs in a bowl.
Roll the meat in the spices, so that it's totally covered from all sides.
Cut the onions in half rings.
Place the onions on the bottom of the pressure cooker and put the herbed sirloin on top.
Add the Marsala wine.
Cover with lid, close properly and set it on high.
When the pressure cooker starts to "sing" and emit steam, lower the temperature to the lowest temp. available.
Cook for 20 to 30 minutes.
Then, take the pot off the stove and put it in your sink.
Let the cold water run over the lid. After about 30 seconds all the steam will escape, the pressure will be released and you will be able to open the lid without any injuries (*sarcasm*). Be very careful as the steam in the pressure cooker is at great pressure.
Let the meat rest for at least 15 minutes.
You can serve it right away or let it cool and then cut it and serve it cold.
To serve hot, cut into slices and pour a couple of tablespoons of the liquid that was left over from cooking. The onions can be put on top of the slices of meat if desired.