While the cauliflower bakes, coat the bottom of a small pot with oil and place it over medium heat.
Add the white parts of the scallions, garlic, ginger, serrano pepper, and celery. Sauté everything for about a minute, until very fragrant.
Stir in the broth, soy sauce, tomato paste, maple syrup, rice vinegar, sambal oelek or sriracha, and black pepper.
Bring the mixture to a boil, lower the heat, and let it simmer for about 10 minutes, stirring occasionally.
Stir the water and cornstarch together in a small bowl or cup, then stir the mixture into the broth mixture. Bring it back up to a simmer and let it cook for about 1 minute, until it thickens up a bit.