A vegetarian for more years than I care to mention (and now post-vegetarian), and quasi-athletic, I needed to educate myself on quality protein sources. And I needed to learn how to cook without meat, having grown up in a household where dinner was top-heavy with meat. I'm sure this recipe is derived from Jeanne Lemlin's Vegetarian Pleasures (early vegetarian, i.e. lots of dairy), but I have made it so often it has morphed into my style: while I'm categorizing it as a soup, it could easily qualify as a stew.
I've downplayed the spices because generally I like more ginger and warming spices, but you can modify this any way once you have the basic parameters down.
Heat olive oil in heavy saucepan, add onion, garlic, ginger and stir until onion and garlic are wilted, then stir in the garam masala and cumin for an additional minute.
Add water, lentils, potatoes, asafoetida, and tomatoes with their juice. Bring to a boil, then simmer with lid slightly ajar for 1.5 hours, checking occasionally to stir and add water if the lentils have absorbed the water quickly. You might want to schmush the lentils a bit to help break them down further: in the end the consistency should be thick, and you shall see, upon cooling and/or re-heating, the soup will appear to be almost solid, more a stew than a soup.
When the potatoes and lentils are thoroughly cooked, add some salt and pepper to taste with the cleaned and chopped spinach, let cook to wilt, and it's done.