NOTE: At Flip Sigi, we serve this soup over braised short rib and garlic fried rice.
In a large heavy-bottomed pot, heat oil on high until shimmering. Add onions, garlic, and half of tamarind soup base to pot, stir until well-combined, and begin to caramelize onions.
Once onions are golden in color, add squash, celery root, and carrots to pot. Cook over high heat, stirring constantly, for 8 minutes. Add water to pot to deglaze, scraping up any brown bits from the bottom, and add diced tomatoes and remaining tamarind soup base to soup. Stir well to combine, then bring to a boil. Reduce heat to low and simmer for 15 minutes before serving.