Pour the vegetable oil into a pot and set it under medium heat, allowing the oil to heat up (see note).
Add the mustard and fennel seeds to the hot oil and allow them to sizzle and start popping.
Once the popping sound gets a bit more intense, add the onion and sauté until it softens and develops a golden color.
Add the garlic, ginger, and chilli pepper. Stir and sauté for about 40 seconds.
Stir in the ground coriander seeds, cumin powder, turmeric powder, and smoked paprika. Stir and let the spices release their inner flavors, which should take about 1 minute.
Stir in the tomato paste, followed by tomato and 1 teaspoon of salt. Let the tomato juices deglaze the pan and stir to incorporate the salt into the mixture.
Add the sweet potatoes and the washed red lentils. Stir to mix them with the flavor base.
Pour in enough hot water to cover all the ingredients. Bring to a boil, then cover the pot and reduce the temperature to under medium heat. Let it simmer for 30 minutes, stirring occasionally.
Once the sweet potatoes soften, stir in the garam masala and 2 teaspoons of salt. Mash some of the lentils and sweet potatoes with a wooden spoon for a mushier texture.
Finally, stir in coconut cream, and add fresh cilantro. Cook for an extra 5 minutes for the flavors to assimilate and turn off the heat.
Serve the dahl with a side of basmati rice and naan bread, drizzled with a bit of yogurt. Enjoy!