In a small wide bottomed saute pan, heat the oil, add the onion, celery, and carrot. Cook for 2 minutes. Add the wine and reduce for 1 minute. Add the stock, and bring to a boil, reduce to a simmer and add the fish fillets. Cover and simmer for 4 minutes. Uncover and carefully remove the fish, and place in a shallow bowl. Turn the heat up and reduce the liquid for 2 minutes. Add the spinach, thyme, tomatoes, and butter. Adjust the seasonings, and serve over the poached fish fillets.