Chop the onion, cut the pepper into small pieces and slice the mushrooms. Immediately drizzle the lemon juice over the mushrooms to prevent discoloration.
Cut the chicken breast fillet into bite-sized pieces and fry them in a little olive oil for a good 5 minutes. Remove, season with salt and pepper and set aside.
Fry the mushrooms in the remaining fat for about 3 minutes, season with salt and pepper, also remove. Heat some more oil and sweat the onions in it until translucent. Squeeze the garlic. Mix in the pepper pieces and sauté for 3 minutes.
Add the tomatoes and their juice, season with salt, pepper and oregano. Let everything simmer for 10 minutes, then add the chicken and the mushrooms again and cook everything over a low heat for another 10 minutes.
Variation: Instead of chicken breast fillet, you can also use chicken legs.