I haven't actually made powder, but I have dried onion successfully. I've copied the following information from my dehydrator instruction book.
Garlic: Peel, finely chop, spread on solid sheets and dry at 35°C. Store dry garlic as it is or powdered. To make garlic salt, blend one part garlic powder with four parts of salt.
Onions: Remove skins, tops and root ends. Slice into 5mm slices hours or rings and avoid drying with other vegetables due to strong
odour. Excellent added to soups, salads, or cooked dishes. Store as dried and only powder or flake as required. Dry 6-10 hours at 50-55C
Hope that helps
Stacey
