Place the beetroot and potatoes in separate bowls, each with enough water to cover. Set aside to soak until you need them.
Heat the oil in a large frypan over medium-high heat. Cook the lentils, mustard seed, cumin seed and red chillis in the hot oil until the seeds begin to splutter, about 30 seconds. Stir the asafoetida and curry leaves into the mixture and cook another few seconds.
Drain the beetroot and potatoes and add to the frypan. Cook and stir until they begin to brown, about 5 minutes. Reduce heat to medium-low, cover, and continue cooking until the potatoes and beetroot are soft, about 20 minutes more. If there is excess water in the pan, increase the heat to high and cook until the water evaporates completely. Stir the red capsicum and coconut into the mixture and serve immediately.
Urad dal is derived from black lentils and can be found in stores specialising in Indian cooking.
Asafoetida is a powder primarily used in Indian cooking and can be found at specialty stores.