Rinse and drain dhal.
Place dhal in a medium sized pot with ginger, garlic, dried chilies, curry leaves, ground turmeric, and 2 cups (480ml) water. Bring to a boil.
Season with salt, reduce heat and let it simmer for about 20 minutes until dhal is soft.
Remove from heat. Using an immersion blender, pulse several times to desired consistency.
In a fry pan, heat butter. Add mustard seeds. When it crackles, add cumin seeds and onion. Fry until onion is slightly brown, about 3 minutes.
Add jalapeno peppers and tomatoes. Cook for 2 to 3 minutes.
Transfer fried ingredients to the pot with cooked dhal. Place pot back onto stove and cook for 5 minutes. Turn off heat.
Serve hot with roti or rice.