Heat a large heavy pot over medium heat. Add olive oil, then add onions and cook, stirring often, for about 5 mins, or until tender. Stir in ginger, garlic, and chilli and cook for about 1 min, or until fragrant. Add ground coriander, cumin and cayenne, if using. Stir often for about 5 mins to toast spices.
Stir in peas and enough water to cover, about 6 cups. Bring to a simmer over high heat, then reduce heat to medium-low, cover, and simmer gently, stirring occasionally, for about 45 mins, or until peas are tender.
Add cauliflower, cover again, and cook, stirring occasionally, and adding more water as needed to adjust thickness to your desired consistency, for about 35 mins, or until cauliflower is falling apart and mixture thickens slightly. Add spinach, and stir until wilted.
Spoon dal into bowls or onto platter with rice. Sprinkle with fresh coriander and serve.