To a large pot on medium-high heat, add the oil. When hot, add the cumin, coriander, and mustard seeds, stirring for 30 - 60 seconds to toast the spices.
Then add to the pot the onion, and cook until translucent, about 3 - 5 minutes. Add splashes of water as needed to deglaze the pot.
Add the garlic and ginger, and cook until fragrant and lightly golden, about 2 minutes.
Add the bouillon cube, garam masala, turmeric, and cayenne, and cook while stirring for 30 - 60 seconds longer.
Add the potatoes and water. Cook until tender, about 15 minutes.
Add the peas, chickpeas, and coconut milk. Cook until warmed through.