Place pepper flakes, cumin, and coriander in a spice grinder; grind until fine. Transfer to a small bowl, and stir in garlic powder, pepper, and mustard until combined. Set aside.Advertisement
Heat a 4-quart pot over medium-high heat, and add oil. When oil is hot, pour in popcorn kernels. Cover, and keep pan over heat until first kernel pops. Shake pan gently, close to burner; continue shaking until kernels stop popping, 4 to 5 minutes.
Bring lime juice to a boil in a small skillet over medium heat. Reduce heat; cook until liquid is reduced to 1/2 tablespoon. Add butter, and swirl to melt; add spice mixture. Drizzle over popcorn while stirring to coat evenly. Season with salt, and toss. This popcorn is best eaten the day it is prepared.
The spice mixture can be made in advance; store it in an airtight container in a cool, dark place up to six months.