Mix the flour, egg and a pinch of sea salt until you have a smooth dough, adding a splash of water, if needed. Knead on a flour-dusted surface for 2 minutes, then cover and pop into the fridge. Trim the tough end off the broccoli stalk. Remove the florets, halving any larger ones, and chop the remaining stalk into 2cm chunks. Put one third of the florets aside, then cook the rest with the chopped stalk in a pan of boiling water for 8 minutes. Drain and mash well, then finely grate in the cheese, finely chop the chives and add half, stir in the soured cream, season to perfection and leave to cool. Peel and finely slice the garlic. Divide the dough into eight, then roll out each piece into a 14cm circle, dusting with flour as you go. Divide up the filling on one side of each circle. Lightly brush the exposed pastry with water, then fold it over the filling, twisting along the edge to seal, like in the picture.
Put a large non-stick frying pan on a medium heat with ½ a tablespoon of olive oil, the pierogi and the reserved broccoli florets. Pour in 1cm of boiling kettle water, cover and boil for 4 minutes. Uncover and fry the pierogi and broccoli on one side only for 4 minutes, or until the water has evaporated and the bases are golden. Meanwhile, put 1 teaspoon of oil and the garlic into the pan you used to boil your broccoli, stir until lightly golden, then pour in the tomatoes, simmer for 2 minutes and season to perfection. Serve it all together, sprinkled with the remaining chives.