Split the breakfast roll horizontally and lightly toast cut sides in a dry skillet or toaster until golden, about 1–2 minutes. Butter the cut sides with the unsalted butter and set aside, toasted-side up.

Cook chorizo

Heat vegetable oil in a small nonstick skillet over medium heat. Add the fresh chorizo, breaking it up with a spatula. Cook until browned and cooked through, about 5–6 minutes, stirring occasionally. Transfer cooked chorizo to a small bowl and wipe skillet clean if it has excess fat.

Scramble eggs

Crack eggs into a small bowl, add kosher salt and black pepper, and whisk until combined. Return skillet to medium-low heat and melt a small pat of butter (or use a little oil). Pour eggs into skillet and cook gently, stirring frequently, until soft-curd scrambled and just set, about 2–3 minutes. Remove from heat.

Assemble filling

Fold the cooked chorizo into the scrambled eggs until evenly distributed. Sprinkle the shredded cheddar over the hot egg-chorizo mixture so it melts slightly, about 30 seconds. If using cilantro, fold most of it in now and reserve a pinch for garnish.

Prepare avocado

Scoop the flesh from half an avocado and mash lightly with a fork in a small bowl. Season with a pinch of salt and a squeeze of the lime wedge (if using). Spread the mashed avocado on the bottom half of the toasted roll.

Build sandwich

Spoon the egg-chorizo-cheddar mixture onto the avocado-spread bottom roll. Drizzle the salsa over the eggs (about 2 tablespoons) and add the sour cream on top or on the top roll to prevent sliding. Place the top roll over the filling.

Finish and serve

Press gently, cut sandwich in half if desired, and garnish with remaining chopped cilantro. Serve immediately while warm.