I will second that site. But I disagree with a bit of Chameleon's advice.

The first seed you plant will never set seeds. So if its something with a worthless crop, like mandrake, use cheap soil. If it is something with a useful or salable crop, like krakka roots or mimmet gourds, use G 3 Shroud soil to increase the number harvested. it its something where HQ sells at a premium, like chives, use G 3 La Noscean soil to get HQ.

Always go for a useful crossbreeding when you can. There are 23 known crossings that make T. onions. Only 12 or 16 give decent odds though. There are 18 crossings using T. onions, some of which produce useful and salable crops.

Because T. onions take 10 RL days to mature and most other crops 5 or 6 days, you can run two companion plantings with them. Useful for seed production.