1. Roast the bones in the oven, which will cause the fat to melt away and get a brown colour.
2. In a large pot, fry sliced celery, carrots and onions. Add the bones and the tomato paste and mix well. Fill the pot with water and lower the flame to low, allowing everything to slowly simmer for 4 to 5 hours.
3. After it is done simmering, strain the liquid and reduce it until it is thick and a dark brown colour.