Heat the olive oil in the skillet over medium heat. Add the onion and sauté 4–5 minutes until softened and translucent. Stir in the garlic and cook 1 minute until fragrant.
Add the Brussels sprouts; cook 5–6 minutes, stirring occasionally, until tender and lightly caramelized. Add zucchini, cumin, za’atar, salt, and pepper; cook 2 minutes, stirring to toast the spices and drive off excess moisture.
While the vegetables cook, mash the cannellini beans in the small bowl to a coarse purée. Stir the beans into the skillet to form a thick, cohesive base; cook 2 minutes.
Add the spinach in handfuls, stirring just until wilted, 1–2 minutes. Reduce heat to low. Use a spoon to make three evenly spaced wells in the mixture.
Crack one egg into each well. Cover with the lid and cook 4–6 minutes, until whites are set and yolks are still runny (cook 1–2 minutes longer for firmer yolks).
Remove from heat. Sprinkle lemon zest, cilantro, and dill over the top. Arrange avocado slices and serve hot from the skillet.