Pre heat oven to 180c ff / 355f.
Lightly prick each potato a few times with a fork, and bake for 1 hour or until tender.
Meanwhile, to make the mornay sauce melt butter in a small saucepan, add bacon and cook stirring over medium heat until bacon is cooked. Remove bacon with a slotted spoon and set aside, leaving bacon flavoured butter in the saucepan.
To the same saucepan add flour and continue to stir over low heat for 1 - 2 mins until the flour starts to sizzle and bubble, it will look grainy and get lighter in colour - then remove from heat. Add crushed / minced garlic and stir well.
Take the slightly cooled flour mixture, and still off the heat, add 2-3 tbsp. of milk and mix thoroughly to combine. Once the first addition of milk is completely incorporated, add another 2-3 tbsp. and mix thoroughly to combine.
Repeat this step twice more - at this stage you should have what resembles smooth single cream - you can now add the remaining milk, mixing to combine completely.
Now place the saucepan over low heat and stir continuously for about 4 minutes, or until your sauce is thick, then add chicken, reserved bacon and parmesan, mixing well to combine - for more tips on making a lump-free, smooth white sauce that doesn't burn at the edges, see my notes in this previous post. Once the potatoes are cooked, remove them from the oven. Hold each one with a set of tongs and using a sharp knife split them in half, not quite all the way through to the base. Brush the potatoes with a small amount of butter.
Spoon some of the mornay sauce into the middle of each potato, then place slices of avocado on each, then top with remaining mornay sauce.
Sprinkle with extra parmesan cheese and grill / broil for 1 - 2 mins (see notes) or until browned on top.