Place chicken gizzards in a large saucepan and add enough water to cover them. Cover and bring to a boil. Boil over medium-high heat for 10 minutes. Drain, reserving water, and chop into bite-sized pieces.

Cook rice according to package directions using gizzard water.

Heat oil in a large skillet over medium heat. Sauté onion, mushrooms, and garlic until onion is tender. Add gizzards and sauté for 5 more minutes. Stir in chicken broth, onion soup mix, and tomatoes. Bring to a simmer and cook until reduced by half, 10 to 15 minutes. Stir in rice, heat through, and serve. Season with salt and pepper to taste.