In a nonstick sauté pan or well-seasoned cast-iron skillet, melt butter over medium-low heat, and add onions. Cook, stirring, until the onions start to sizzle, about 2 minutes. Add kimchi and kimchi juice, and stir until it comes to a boil, about 3 minutes. Add Spam, and cook until sauce is nearly dried out, about 5 minutes.
