Sunny but not that warm here.

Last year I grew my pots in tubs and covered the top with grass clippings and they didn't turn into a mat, they just helped with water retention. Make sure you haven't used lawn weed and feed until the lawn had had 3-4 mowings.

Get one that has plenty of compost in it and don't buy cheap as that is all the plant has to grow in. I don't use them but I do use those green tomato plant plant pots that have a central planting pot set in an outer watering space. You water in this area then put your feeding solution in the inner one where the plant is.

we started like you in January in propagators now planted out and doing ok i would guess about 8-10 inches high mostly in rows about 4 inch apart 6 inch between rows some [red baron] are in groups of about 6 dotted around other plants  our main white onion is Ailsa Craig which normally gives us large onions that store well we are just using the last now. good luck.

After buying Miracle Grow a few weeks ago, I vowed never to buy it again so I've been looking for an alternative. A friend suggested I try Clover Irish peat mpc instead, so after a bit of googling I found a local allotment society near me and picked up half a dozen bags at the princely sum of a fiver a bag (60l)!
Wow!! what a difference!! it's so fine with no 'bits' in it and I could quite easily use it for seed sowing without even sieving it. As peat free compost is being banned by the end of the year, I'm now looking for a company that can supply & deliver me a pallet load that I can store for future use!