This is how I grew my calabrese / broccoli and very nice it tasted as well.
Sowed January 2 in an unheated greenhouse. Transplanted March 19 into individual pots. Planted outside April 30. First harvest June 18, final harvest June 22.
The harvests were quite close together so a point to remember for next year is to sow small batches but 3 weeks apart so the harvest lasts over a longer period of time.
January: Seed sown, two per station then remove the weakest seedling.
March: Transplant into individual pots, keeping them in the greenhouse.
End of April/Beginning of May: Plant in their final growing positions in the open ground.
June: Harvest and enjoy.