Heat olive oil in a sauce pan. Add white onions, celery, garlic, pumpkin, salt and chilli flakes. Fry the ingredients stirring continuously till onions turn transparent. Later add water and cover the saucepan and allow it to cook for 20 minutes.
Strain the stock through a fine sieve, pushing down on any solids that remain with the back of a spoon to extract all the liquid. Rinse the saucepan and pour the stock back into the saucepan and boil for 10 more minutes. Put off from the heat and add lemon juice.
Cut the lid off the pumpkin and scoop out the insides to use it as a serving pot. Season the inside of the pumpkin with some olive oil and salt and bake in 180 degree C oven for 10 minutes.