Note that that 'instructable' does not show any pictures of onions grown by this method. You will get onion leaves but as a way of producing bulb onions it's highly suspect. 'You can theoretically create an endless supply of onions without ever having to buy bulbs or seeds,' This is nonsense imo. gator5774 if you seriously want to grow usable onions, rather than indulge in a a fun experiment out of curiosity, buy seed or sets and find out how to do it properly.

This just popped up on my list, which is strangely delayed. Again, no biggie. If they bother you, pluck them out or turn them under (without disturbing a plant if present!) Otherwise, you can ignore them if you want to. The only slight warning is that if they're indoors (at this point, likely) keep children and pets away from the mushrooms. Without positive ID, we can't tell if they're toxic or not. And if you're allergic to fungal spores (I sure am), you may not want to allow the mushrooms to remain. Most mushrooms don't bother me, however.

There's no need to grow a whole flat of onion transplants. Onion seeds can be sown inside in most any small clean container. I like plastic containers that held store bought strawberries or blueberries since they have lots of small holes for good drainage. Fill it about 3/4 full with soiless seed starting or potting mix, add the seeds, spacing them about 1/4 to1/2 inch apart in all directions and then cover with 1/4 to 1/2 inch of addition mix. Keep the mix moist but not soggy and you should have tiny onions before long. They take longer than many seeds to sprout....maybe as long as 10 to 14 days depending on conditions. Mine do not all come up at once but after a week or two the slow pokes do show up and I have a small tray with lots of plants. After they gain some size I'll transplant them. I'll start in late winter and grow them inside for 2 or more months. It is hard to keep direct seeded onions weeded since they grow slowly at first. I much prefer to transplant them. I'm not sure what the timing would be for you to grow bulbing onions in your area, but you can always use them as green onions and figure you learned about growing them to transplant size and will be know that skill for next season.

When I have bark in my soil this seems to happen and as you said, is harmless. I still pluck them out as I dont want them attracting other pests.