Preheat your oven to 400°F (200°C). Cut the acorn squashes in half from stem to tip and scoop out the seeds. Brush the flesh with olive oil and sprinkle with salt and pepper. Place the squash cut-side down on a baking sheet and roast for 30 minutes until the flesh is tender when pierced with a fork.
While the squash is roasting, heat a small splash of olive oil in a skillet over medium heat. Add the onion and sauté for 3-4 minutes until translucent. Add the minced garlic and cook for another minute until fragrant. Remove from heat and transfer to a mixing bowl.
In the mixing bowl with the onions and garlic, add the breadcrumbs, Parmesan cheese, half of the mozzarella cheese, dried oregano, and red pepper flakes. Stir until the mixture is well combined. If the mixture seems too dry, you can add a teaspoon of olive oil.
Once the squash is tender, remove it from the oven and carefully flip the halves over so the flesh side is facing up. Divide the cheese and breadcrumb mixture evenly between the four squash cavities, pressing down slightly to pack it in.
Top each stuffed squash with the remaining mozzarella cheese. Return the baking sheet to the oven and bake for an additional 10-15 minutes, or until the cheese is melted, bubbly, and slightly golden brown. Garnish with fresh parsley before serving.