starting seedlings

So I'm starting some onion seedlings soon and eventually my other plants. I don't have access to my compost as it's frozen solid. Next year I'll store some in my basement. I know that a lot of the seed starting bagged soil mixes are sterile what should I use to make said mix no longer sterile? So my plants have a better chance of survival, I feel nothing should grow in a sterile environment including humans as it is not healthy.

The reason for a sterile seed starting mix is to avoid anything that might interfere with the growth of the plant (weed seed or disease). I don't use a sterile mix though...I just make sure i know my ingredients. Most seeds, i think, have everything they need to sprout in the right moisture and light environment...it is when they are growing the need food
My compost froze this year also but we have had enough warm days to pick at it for potting. I don't worry much about feeding them until they are in the ground and growing faster.

I'm sure others will be able to better help with your question

I think so I guess my concern was more of starting in sterility then shocking them with something non sterile...kind of like the equivalent of putting a baby in a bubble then at age 1 sticking the baby in a germ filled day care. With no immune system to speak of the baby/plant is at a great disadvantage.

The baby analogy isn't great because the baby has had 9 months in a bubble, so to speak. It gets immunity from it's mother and yet, being thrust out into the world is dangerous.

I start my seeds in coco-peat. I'm not sure if it's sterile but it doesn't have soil born diseases. The plants don't need food for a while anyway. To prevent damping off I have an oscillating fan blowing across them on a timer once they sprout. For food, I take water from my aquaponics set-up.

The baby is inoculated with microorganisms at birth. Vaginal birth vs cesarean inoculates the child with vastly different microorganisms, and has life-long health ramifications.

Sterility is a fallacy, even in sterilized media. Microorganisms are everywhere, and will soon show up in 'sterilized' media.
I'd recommend mixing in compost, earth worm castings, or a tea made from such. And some mycorrhizae.