Heat oil in large pot over high heat. Sprinkle bones with salt and pepper. Add bones and onion to pot. Saut’ until bones and onion are deep brown, turning often, about 20 minutes. Add 4 quarts water and bring to boil. Reduce heat to medium-low and simmer uncovered until stock is reduced to 3 to 4 cups, about 3 1/2 hours. Strain, discarding bones and onion. DO AHEAD: Can be made 3 days ahead. Refrigerate uncovered until cold, then cover and keep chilled. Spoon off and discard all fat before using.