Melt bacon grease in the same skillet over medium-high heat. Add diced potatoes and cook, stirring occasionally, 8 to 10 minutes. Add onion and serrano pepper and continue cooking and stirring until potatoes are fork-tender, onion is translucent, and serrano pepper is soft, 2 to 6 minutes. Add eggs and chorizo; stir until cooked and completely incorporated into potato mixture, about 5 minutes. Season with salt and pepper.