Preheat the oven: Preheat your oven to 400°F (200°C).
Prepare the squash: Wash and dry the acorn squash. Carefully cut each squash in half from stem to bottom. Scoop out the seeds and stringy pulp.
Season the squash: Brush the cut sides with olive oil and season lightly with salt and black pepper.
Roast the squash: Place the squash halves cut-side down on a baking sheet. Roast for 30–40 minutes, or until the flesh is tender and easily pierced with a fork.
Brown the beef: While the squash cooks, heat a large skillet over medium heat. Add the ground beef and cook, breaking it apart with a wooden spoon, until browned and cooked through. Drain excess grease if necessary.
Cook the vegetables: Add the diced onion and bell pepper to the skillet. Cook for 4–5 minutes, until softened.
Add garlic and seasonings: Stir in the minced garlic, paprika, thyme, sage, salt, and black pepper. Cook for about 1 minute, until fragrant.
Add optional ingredients: Stir in the cooked rice or quinoa and additional vegetables such as spinach or mushrooms. Cook until everything is hot and well combined.
Stuff the squash: Remove the roasted squash from the oven and carefully turn the halves cut-side up. Divide the ground beef mixture evenly among the four squash halves.
Add cheese: If using, sprinkle the shredded cheese over each stuffed squash half.
Bake again: Return the stuffed squash to the oven and bake for 8–10 minutes, until the filling is hot and the cheese has melted.
Garnish and serve: Remove from the oven and let the squash cool for a few minutes. Sprinkle with fresh parsley and serve warm.