Preheat the oven to 200 degrees Celsius. Rinse the zucchini and cut it lengthwise in half. Use a spoon to carefully scoop out the flesh, leaving two sturdy hollowed-out halves. Then mince the half onion into small pieces.
Heat a frying pan and cook the lean ground beef until browned and crumbly. Add the minced onion and cook briefly until the pieces become translucent. Then stir the teaspoon of tomato paste into the beef and heat through for a minute. Finally, fold the tablespoon of pine nuts into the filling and remove from heat.
Place the hollowed zucchini halves in a baking dish or on a baking sheet. Divide the warm beef mixture evenly between the two halves and press the filling down lightly. Slide the dish into the preheated oven and bake the zucchini for twenty minutes until cooked through.
Remove the dish from the oven and let it cool for a minute. Place the stuffed zucchini on a plate and serve immediately.