Preheat oven (electric cooker: 225 °C/circulating air: 200 °C/gas: see manufacturer). Unroll the pizza dough and place on a baking tray with baking paper. Grate cheese and spread evenly on the dough. Bake in a hot oven for 15-20 minutes.
In the meantime peel and halve the onions and cut them into fine strips. Wash, quarter, seed and dice the tomatoes. Cut avocados in half lengthwise and remove the core. Remove the flesh from the skin, dice and immediately mix with lemon juice.
Pour the sour cream into the freezer bag.
Take pizza out of the oven. Sprinkle with onions and top with tomatoes. Spread avocado on top. Season with salt and pepper. Cut a small corner from the freezer bag and spray sour cream in strips on the pizza.
Cut into pieces and serve immediately.