5 pounds beef bones
5 pounds chicken bones
1 half-bunch celery
1 carrot
1 - 2 onions
1 bay leaf
2 - 4 cloves garlic
1 pinch thyme
1 cup tomato puree

Brown beef bones and boil. Keep covered with water for 8 to 10 hours. Strain, reserving liquid. Put beef bones in pot with chicken bones, vegetables and seasonings. Cover with water and cook 4 to 5 hours. Strain. Reserve liquid. Combine the second liquid with the first. Add tomato puree. Strain. Reduce until liquid will coat a spoon. Skim throughout reduction.

Note: Reduction works best in a tall pot on low heat. Refrigerate between steps to bring fat to top and strain. Leftover demi-glace may be frozen in ice cube trays for future use.