30–32 ounces of frozen whole leaf spinach (3 10-oz boxes or 2 16-oz bags) note: you can use chopped spinach, but I have a slight preference for the quality of whole leaf spinach.
1 large yellow onion, peeled, halved lengthwise and thinly sliced crosswise
4 tablespoons vegetable oil
1 tablespoon plus 1 teaspoon black mustard seeds (or substitute yellow mustard seeds)
1 tablespoon minced garlic (2 large cloves)
1/4 teaspoon cayenne pepper (use a pinch more if you like a lot of heat)
1 1/4 teaspoons of salt plus more, to taste
1 1/2 pounds russet potatoes, peeled and cut into 3/4-inch to 1-inch cubes
Freshly ground black pepper, to taste
Fill a large saucepan halfway with water and bring to a rolling boil. Add frozen spinach to the pot and cook, stirring, for a minute or two, just until spinach is defrosted and tender. Drain and rinse under cold water to stop the cooking process. Squeeze the spinach in your hands to wring out most of the water. Chop the spinach coarsely and set aside.
Heat oil over medium heat in a large heavy skillet or Dutch oven with a lid. Add mustard seeds. Leave them for just 30 seconds or so. As soon as they start to pop, add the onions and garlic to the pot. Cook, stirring, for 2 minutes, until the onions are slightly softened. Add potatoes and cayenne pepper. Toss to combine with onion mixture.
Add spinach, salt and 1/4 cup of water. Turn heat up to get the water sizzling. Cover pot and lower to a very gently simmer. Simmer gently for 40 minutes or until the potatoes are tender. Give everything a stir every 15 minutes or so, adding a bit more water, if the pan looks dry.