There are lot of hen houses. For example, one is just opposite to charlatan’s stall. The “Rob” interaction button may not appear correctly. Just try pointing to the wooden ladder like thing at hen houses. Also look for Hens 
Now, about the potion, i will not spoil this for you. But as close i can get is,
1 - Need Three ingredients
2 - In story they drink wine, so you need wine ofcourse.
3 - One flower is red and grows in fields.
4 - One is Prickly.
5 - Last one carries a Saint’s name. (This is the easiest to guess)
6 - Just grind them one by one. Put them in the cauldron and boil for one turn. DONE!!
Also to throw eggs in to the bath house well, you need to spoil them first. FYI, you will get “Throwing Eggs into bathhouse well” quest popup after you talk to the grocery store owner.