Heat a little olive oil in a large saucepan over medium-high heat. Add the chopped onion, celery, and garlic, and sauté for 3–4 minutes, stirring frequently, until softened and lightly golden.
Add the ground beef and cook, breaking it up with a wooden spoon, until browned and no longer pink.
Stir in the diced tomatoes and beef broth. Season with salt and pepper, then reduce the heat and simmer for 25–30 minutes, stirring occasionally, until the sauce thickens.
Meanwhile, prepare the bell peppers. Carefully slice off the tops, keeping the stems attached to create little lids. Remove the seeds and membranes. Using a small, sharp knife, carve triangular eyes and noses into the peppers, adding mouths to create jack-o'-lantern faces.
Once the meat mixture is ready, transfer it to a large bowl. Add the cooked rice and stir until evenly combined.
Spoon the filling into the prepared peppers and sprinkle the shredded Gouda over the top. Replace the pepper lids and serve.
Optional: If you prefer softer peppers, bake them in a preheated oven at 350°F (180°C), using the convection setting, for 15–20 minutes, or until softened to your liking.