Spicy Sauteed Spinach

2 Tbsp. ghee (clarified butter) or vegetable oil
1 bunch scallions, coarsely chopped
2 dried chilies de a'rbol or ½ tsp. crushed red pepper flakes
2 garlic cloves, finely chopped
2 tsp. yellow mustard seeds
4 bunches flat leaf spinach, stemmed, very coarsely chopped
Kosher salt, freshly ground pepper

Heat ghee in a large skillet over medium high heat. Add scallions, chilies, garlic, and mustard seeds. Cook. Stirring often, until garlic begins to brown and mustard seeds pop, about 1 minute. Add spinach to skillet by the handful, allowing it to wilt between additions. Cook, tossing often, until spinach is tender, 5-8 minutes. Season with salt and pepper.