Place a large saucepan or pot over medium heat and add the vegetable oil.
Once shimmering, add the chopped yellow onion and sauté until soft and translucent, about 3-4 minutes.
Add the whole mustard seeds.
When they begin to pop and crackle, add the minced green chili and crushed red chilis.
Stir for 30 seconds until fragrant.
Tip in the farina and, stirring constantly, roast for 3-4 minutes. The farina should become highly aromatic and turn a very light golden color.
Do not let it brown.
Reduce the heat to low. Carefully and gradually pour in the 5 cups (1200 ml) of hot water while whisking continuously to prevent any lumps from forming. The mixture will sizzle and steam vigorously.
Once all the water is incorporated, stir in the sliced green onions, diced carrots, diced tomato, and salt. Increase the heat to medium and bring the mixture to a gentle simmer.
Reduce the heat back to low, cover the pan, and cook, stirring every 5-7 minutes to prevent sticking at the bottom. The upma is ready when it is very thick, has pulled away from the sides of the pan, and the carrots are tender. This will take 45-60 minutes due to the high water ratio, resulting in a soft, porridge-like consistency.
Garnish generously with fresh cilantro and serve immediately with lemon wedges on the side for squeezing over.
Roasting the farina (sooji) until it's aromatic is the most crucial step. This prevents the upma from becoming sticky or pasty and adds a wonderful nutty flavor.