While potatoes are cooking, preheat oven to 425 degrees F (220 C).
Set a large oven-safe skillet (see notes) over medium high heat and add oil. Once the oil is shiny, add onions and garlic and sauté until fragrant and slightly browned, about 2-3 minutes.
Add celery, carrots and herbs and sauté until carrots are softened. Add the tomato paste, roasted cumin powder and garam masala.
Drain the soaked lentils and add them to the skillet along with stock, salt and black pepper and stir. Bring to a low boil. Then reduce heat to simmer. Cover and continue cooking until lentils are tender (30-35 minutes). At this point, I set the timer for 20 minutes. After 20 minutes, add the frozen veggies, stir, cover and set the timer for 10 more minutes. Check the lentils for doneness after the timer goes off, they should retain some bite and should not be mushy. Turn the heat off.
Once the potatoes are cooked, drain and peel them (see notes), and add back to the pot. Add yogurt, butter, turmeric, chaat masala 9if using), salt and black pepper.
Mash everything together until smooth and creamy. Give a taste check for seasonings.