Things this Vegetable Growing Cheat Sheet will show you 1. When to plant 2. How far apart to plant seeds 3. What needs propagating 4. What needs to be in a greenhouse 5. What size pot to plant in 6. Distance to thin seedlings out to 7. Germination & maturation times 8. Which pests to look out for 9. What veg works best together 10.When to harvest!
This article titled "How To Grow Sprouted Onions" at Alternative Gardening talks about exactly that. I had no idea. All this time, if I didn't eat an onion in time, I would simply plant the whole thing as-is in the soil.