Melt the butter in a small skillet over medium heat. While the butter melts, beat the eggs with the salt and black pepper in a small bowl.
Pour the eggs into the skillet and cook, stirring constantly and scraping the bottom of the pan, until just set. Transfer the eggs to a bowl.
Toast the brioche bun in a 350°F (177°C) oven or toaster oven until lightly golden.
Place the cheese on one half of the bun. Top with the eggs, spinach, and tomato slices. Spread the mashed avocado on the other half of the bun.
Close the sandwich, cut in half, and serve warm.
Notes
Prep Before You Start: This breakfast sandwich comes together quickly, so read through the recipe and ingredient notes and have everything ready before you begin.
Layer for Stability: Place the cheese on the warm bun first so it melts slightly, then add the eggs. The mashed avocado helps keep the tomatoes from sliding.
Clean Slices: For a neat cut, wrap the assembled sandwich in butcher paper and slice it in half with a serrated knife.
Alternate to Toasting: For extra richness and a crisp exterior, pan-fry the bread in a little butter instead of toasting it.