Heat olive oil in a nonstick pot over medium heat. Add chicken thighs, Italian seasoning, and garlic; cook until chicken is browned on both sides, about 5 minutes per side. Mix mushrooms and tomato into chicken mixture; reduce heat, cover pot, and cook until mushrooms are softened, about 30 minutes.

Preheat oven to 350 degrees F (175 degrees C). Grease a baking dish.

Drain accumulated juices from chicken-mushroom-tomato mixture into a separate saucepan and transfer chicken-mushroom-tomato mixture to the prepared baking dish.

Pour wine into saucepan with juices and bring to a boil. Add cream and Gruyere cheese; cook and stir until cheese is melted and sauce is smooth, 3 to 4 minutes.

Stir cornstarch and water together in a small bowl until cornstarch is dissolved; mix into cream sauce until smooth and thickened. Pour sauce over chicken-vegetable mixture; top with onion.

Bake in the preheated oven until chicken is no longer pink in the center, about 30 minutes. An instant-read thermometer inserted near the bone should read 165 degrees F (74 degrees C).