Preheat the oven at 400 °F and grease an oven-proof glass dish with some oil.
Cut the peppers in half, lengthwise, and discard the seeds; set aside.
Place them on the prepared pan or dish. Season with salt and pepper and bake for 10-12 minutes, or until slightly softened.
Meanwhile, in a heavy skillet, add oil and heat until shimmering hot.
Add sliced beef together with grated onion and seasonings; Cook stirring occasionally until evenly golden brown, about 6-8 minutes.
Stir in the diced tomatoes and 2/3 of the shredded cheese. Season with a pinch of salt and pepper and take off the heat.
Next, stuff the roasted peppers halves with the steak filling. Top with the remaining shredded cheese on top.
Bake for 10 minutes, or until cheese is melted and bubbly.
Garnish with chopped cilantro leaves and serve with lemon wedges.