Heat the oil in a wide saucepan or wok. Add the mustard seeds and, when they sputter, add the onions and garlic.
Sprinkle on some salt and saute, stirring frequently, until the onions are translucent, about three to four minutes.
Add the brussels sprouts and stir to mix. Turn the heat to low. If you have a heavy lid with a diameter slightly smaller than that of the saucepan, use it to cover the sprouts while at the same time pressing down on them. Otherwise, just cover the saucepan and let the sprouts cook. Stir every three or four minutes to make sure they are roasting evenly.
When the sprouts are lightly golden brown and almost al dente, tender but with a pleasant bite, mix the besan flour in a small bowl with ¾ cup of water, cayenne, turmeric, cumin and salt.
Pour into the brussels sprouts. The besan will start firming up as it heats. Scrape with a spatula, the way you would cook scrambled eggs, and continue to cook another 10 minutes, stirring frequently, until the besan is cooked through and don't have a raw flavor any more.
Check salt, garnish with coriander leaves if desired, and serve hot.