Heat oil in a large stockpot. Add onions and cook until onions start to turn golden brown , about 10 minutes or more. Add the tomato paste and stir into the onions, cooking for about 2 minutes until fragrant. Add all the chopped vegetables to the onions, along with all the rest of the ingredients, including the boxes of broth. Make sure all the vegetables are covered by the broth. If not, add a bit of water. Bring to a boil, then turn down and simmer uncovered at a VERY low temperature (barely bubbling), stirring occasionally, for 1 1/2 to 2 hours. Stock will reduce down to about 6 cups. Do NOT add any salt.