Preheat the oven to 400°F. Fill a 4- to 5-quart pot three-quarters full with water and bring to a boil. Add the broccoli florets and cook 2 minutes. Drain immediately and chop into small pieces. For faster prep time, this can be done ahead.
Quarter the sausage links lengthwise and then chop crosswise into small pieces. Cut off the dark green tops and the root end off of the leek and use the white and light green parts. Split the leek lengthwise and thoroughly rinse under cold water to dislodge any sand or dirt. Chop the leek crosswise into thin slices.
Melt the clarified butter in a 10-inch well-seasoned cast-iron skillet over medium heat. Add the leeks to the pan and cook until softened, about 2 minutes. Add the sausage pieces and brown for 3 to 4 minutes, stirring with the leeks. Stir in the broccoli and spread the vegetables and sausage into an even layer.
In a medium bowl, whisk the eggs together until smooth, and then pour over the vegetables. Spread the eggs evenly over the vegetables with a spatula if needed. Sprinkle evenly with the oregano and pepper.
Place the pan in the oven and bake until the frittata is set in the center and the top is a light golden brown, 14 to 16 minutes.
Remove the frittata from the oven when fully cooked and cut into 4 wedges. Divide the wedges onto 4 plates, top each serving with a quarter of the avocado and serve immediately or at room temperature. Leftovers should be covered and refrigerated for up to 2 days. Reheat in a warm oven or enjoy at room temperature.