Chop 1 medium onion, cut 2 bell peppers into small pieces, and slice 250 g mushrooms. Immediately drizzle 2 tablespoon lemon juice over mushrooms to prevent discoloration.
Cut 600 g chicken breast fillets into bite-sized pieces. Heat olive oil in a large pan over medium-high heat. Fry chicken for 5 minutes until cooked through (internal temperature 74°C / 165°F). Season with salt and pepper and set aside.
In the same pan over medium-high heat, fry mushrooms for 3 minutes. Season with salt and pepper and remove from pan. Add more olive oil to the pan, then cook the onion over medium heat for 3-4 minutes until translucent. Add 2 cloves garlic and bell pepper pieces, then sauté for 3 minutes over medium heat.
Add 400 g tomatoes with juice to the pan and season with salt, pepper, and 1 teaspoon oregano. Simmer over medium heat for 10 minutes. Return the cooked chicken and mushrooms to the pan and cook over low heat for 10 minutes.