01 - Preheat oven to 400°F. Line a baking sheet with parchment paper.
02 - Wash and halve zucchinis lengthwise. Using a spoon, carefully scoop out flesh, leaving about ⅓ inch border to form boats. Reserve half the scooped flesh, chop it, and set aside.
03 - Heat olive oil and butter in a large skillet over medium heat. Sauté onion for 2-3 minutes until translucent. Add garlic and cook for another minute.
04 - Add diced beef tenderloin, season with salt and pepper, and sauté for 2-3 minutes until just browned.
05 - Stir in chopped mushrooms and reserved zucchini flesh. Cook until mixture is dry and mushrooms are golden, about 5-7 minutes.
06 - Remove from heat, then add parsley and half of the mozzarella or Gruyère cheese. Stir to combine.
07 - Place zucchini halves on prepared baking sheet. Sprinkle with a little salt.
08 - Fill each zucchini boat generously with beef and mushroom mixture. Top with remaining mozzarella or Gruyère and sprinkle with Parmesan.
09 - Bake for 20 minutes, or until zucchini is tender and cheese is golden brown.
10 - Let rest for a few minutes before serving. Garnish with extra fresh parsley if desired.