I would say you can sow but can't plant out tender veg plants till the last frost
I've not grown onions from seed so can't advise but sowing most veg seeds in Feb/March unless your soil is warmed up and the plants protected, will need nurturing for several months in small pots either indoors or a heated GH and there is nothing to be gained by sowing early because later sown vegs catch up.
Parsnips also grow a tappered root and don't take to kindly to being started off in a pot as your parsnip wil develop from that root which needs to grow straight down and not be restricted with the bottom of a pot.
Do you have a GH or space in doors to sow? Where do plan to keep the sown seeds until planting out?