Blanch the oxtail pieces in boiling water for a few minutes, then drain and rinse.
In a pressure cooker or large pot, heat the lard, add the onion, paprika and salt - and brown.
Add the oxtail and fry for 10 minutes, stirring occasionally.
Gradually pour in water until a good stock forms.
Add the bay leaves and chili, cover and braise for about 30-40 minutes in a pressure cooker (up to 2 hours in a regular pot), until the meat comes away from the bone.
Before serving, taste, adjust the salt and garnish with freshly cut watercress.