Step 1

Preparation Using damp hands, form sausage into two 1/4"-thick patties.

Step 2

Heat a large cast-iron skillet over medium-high until hot. Cook patties until golden brown on bottom side, about 2 minutes. Turn and continue to cook until other side is browned and cooked through, 2–3 minutes more. Transfer to a plate. Wipe out skillet.

Step 3

Heat butter over medium in same skillet until melted. Cook eggs, undisturbed, until whites are set, edges are golden, and yolks are still runny, 4–5 minutes. Season with salt and red pepper flakes (if using). Remove pan from heat.

Step 4

Spread harissa on 2 muffin halves. Top with feta, patties, eggs, peppers, and cilantro. Spread mayonnaise on remaining muffin halves, then close up sandwiches.