My question is regarding the Creamy Sausage Stuffed Mushrooms; 2 medium onions & 5 garlic cloves sound like an awful lot...

The onions are going to reduce down a lot in the caramelization process. The garlic has to stand up to the flavor of the sausage - if you're worried, you could scale back a bit. In my experience the recipes tested by the food52 test kitchen have worked well as written.