This dish can be a main (vegan/vegetarian also), especially if served with poori or naan. It can also be served as part of a many dish Indian meal.
The recipe is based on a Madhur Jaffrey recipe that I have tweaked a bit for our taste, as well as ingredient availability.
30 to 32 ounces fresh baby spinach
4 tablespoons vegetable oil
1 Tbsp plus 1 tsp brown mustard seeds - or a bit more if you want a stronger mustard flavor
1 large yellow onion, peeled, halved lengthwise and thinly sliced crosswise
1 Tbsp minced garlic
1-1/4 tsp salt plus more, to taste
1-1/2 pounds russet potatoes, peeled and cut into 3/4-inch to 1-inch cubes
Really good pinch of Kashmiri chile pepper (or a pinch of cayenne pepper and a pinch of paprika as a substitute) - use an extra pinch of the hot peppers if you like a lot of heat
Freshly ground black pepper, to taste
Steam the spinach in a large pot until just wilted. Cool and then squeeze out as much liquid as you can. Reserve 1 cup of the liquid for later use.
In a large heavy skillet or Dutch oven with a lid, heat oil over medium heat. Add the mustard seeds and let them cook for 30 seconds or so until they start to pop. Then, add the onions and cook, stirring, for 2 minutes, until the onions are slightly softened. Add the garlic, stir and cook for 1 minute. Add potatoes and Kashmiri powder, and stir to combine.
Add the spinach, salt and 1/4 cup of the reserved liquid. Turn heat up to get the water sizzling. Cover pot and lower to a very gentle simmer. Simmer gently until the potatoes are tender, about 40 minutes, stirring every 15 minutes or so, adding a bit more reserved liquid if the pan looks dry.
Season with salt and pepper to taste. Serve hot.