Heat 2 tablespoons oil in a saucepan. Cook onion for 6 minutes, then add garlic, ginger, turmeric, cumin, and cayenne for 1 minute.
Add stock and potatoes. Simmer for 15 minutes until the potatoes are tender. Stir in coconut milk. Whisk flour with cool water, then add the slurry gradually while stirring.
Simmer for 8 to 10 minutes until smooth and lightly thickened. Season with salt and lime juice.
Fry the Bhajia:
Mix chickpea flour, water, onion, chile, cumin, and salt into a thick drop batter. Rest for 10 minutes.
Heat frying oil to 350 F (175 C). Drop rough tablespoonfuls of batter into the oil and fry for 3 to 4 minutes, turning once, until irregularly golden. Drain well.
Assemble:
Divide potatoes and bhajia among bowls. Ladle over hot soup and add egg, tamarind chutney, coconut-cilantro chutney, cassava crisps, and cilantro to taste. Serve immediately.
Notes
Add the flour slurry slowly and stir constantly so the soup remains smooth rather than lumpy.
Keep soup and garnishes separate for up to three days; fry or refresh crisp elements just before serving.