I've been saving onion skins for a few weeks. What I have in my head is a golden coloured yarn; I'm imagining a ripe wheat field.
The first step is to simmer the fleece in a mordant. This helps the dye to stick to the fibre. Without it, the colour would wash right out. There are lots of mordant options, some affect the finished colour too.
I used alum (potassium aluminium sulphate). The fleece has to be simmered in this for an hour or so.
I was amazed by the depth of colour that came out of the skins when I started to boil them. A very rich reddish brown.
I experimented by dipping locks of the fleece into the dye for various lengths of time. Surprisingly the resulting colour was yellow rather than brown. The time that the wool was in the dye didn't seem to make a difference, but the first lot that went into the dye is certainly much richer in colour. (The lump in the lower left of the picture I held back from the dye and left its natural creamy colour.) It's now drying and I can't wait to start spinning it!