1. Heat oil in a large (at least six-quart) stockpot. Add onions and sauté for five minutes, then add celery, carrots and a pinch of salt. Let the vegetables sweat over low heat for 10 minutes, stirring occasionally.
2. Add potatoes, marrow bones, 12 cups water, salt and pepper.
3. Cover the pot and bring to a boil. Reduce heat to a low simmer and cook with the lid slightly ajar for two to four hours. Skim foam and excess fat occasionally, if desired.
4. Remove from heat and let cool slightly.
5. Remove bones. Pulse three to four times with a hand blender to slightly blend the vegetables.