1Peel and thinly slice the onions. In a large heavy-bottomed pot, add onions along with 2 tbsp of oil over medium-high heat. Stir often for 5-10 minutes until moisture escapes and volume reduces by half. Turn heat to low and stir every 5 minutes for 45-60 minutes until onions are deep brown.
2Roughly chop mushrooms into equal size pieces. Pre-heat oven to 425 degrees Fahrenheit. Pre-heat a large oven-proof sauté pan over high heat, add mushrooms along with 2 tbsp of oil, and sauté for 1 minute before transferring to the oven.
3Stir mushrooms every 5 minutes for 15 minutes until browned. Add tomato paste, toss to coat, and roast for another 3-5 minutes until caramelized.
4Add caramelized mushrooms to the pot with the onions. Deglaze the mushroom pans with water and add the fond to the pot.