The companion (Thrall / Horse / Animal) eats the recommended products in the inventory, but the achievement is not counted.
Single-player mode, Steam.
I tried:
Long “E” to companion and select Inventory // Shot “E” to companion
Not full-HP companion // Full HP-companion
Click button “Give”, “Take” // doble mouse-click on food
Various recommended meals.
With open companion inventory-window // with closed companion inventory-window
Released old companions and put new ones on the ground
Companion in party // without party, in security mode
Hey there 
That journey step completes by “force-feeding” your follower.
So it’s not when they consume food automatically, but when you put a food item in their inventory and while it’s in their inventory click it and click Use
Of course that only works with edible food that your character can also consume and doesn’t work with resources like plant fiber as there will be no Use button for those in the pet inventory.
This doesn’t matter though, the food does not need to be on their diet list, you can give a gazelle a steak if you want to
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Hello. Really, thanks!
I do not know why, but it does not work for me with porridge for slaves. It worked on a pet turtle, “Use” the crab. 
Yea, this one is specifically for pets so it wouldn’t work with regular thralls.
Anyway, glad you managed to get it with the turtle!
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