Halved lengthwise the zucchinis and with a small spoon hollow out the flesh of the zucchini. Be careful not to scrape thru. Rule of thumb when removing the inside flesh, follow the guideline of the seeds. Once the seeds are all removed stop scraping.
In a skillet over medium heat, heat 2 tablespoons of olive oil and cook the onion and the zucchini flesh until the flesh has diminished and some melted.
Meanwhile in a bowl mix the ground sirloin, ricotta, pine nuts, parmesan cheese, chopped parsley. Mix well and season with salt and pepper to your liking. Add the onion zucchini flesh mixture and mix well.
Preheat the oven at 350F. Generously sprinkle salt and pepper on the cavity of the zucchini. Using a heavy bottom gratin dish, fill the zucchini with the meat mixture. Drizzle the remaining 1 tablespoon olive oil over the zucchinis and add 3-4 tablespoons of water to the bottom of the gratin dish.
Bake in the oven for 30 minutes. Serve hot.