Saag Aloo: Indian Spinach and Potatoes

Ingredients

• Freshly ground black pepper, to taste
• 4 tablespoons vegetable oil
• 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 tablespoon plus 1 teaspoon black mustard seeds (or substitute yellow mustard seeds - see notes)
• 1 large yellow onion, peeled, halved lengthwise and thinly sliced crosswise
• 1 tablespoon minced garlic (2 large cloves)
• 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
• A pinch of cayenne pepper (use an extra pinch or two if you like a lot of heat)