Preheat the oven to 300°F.
Toss the onions, garlic, celery and carrot into the bottom of a heavy lidded pot and set the chicken on top. Sprinkle the lot with salt and pepper and toss in a few sprigs of rosemary or thyme or whatever fresh herbs you have, if you have them.
Cover with a tight-fitting lid and bake the chicken for 90 minutes or so, until the juices from the cavity run clear and the joints wiggle in their sockets. Remove the chicken and rest on a plate, covered with foil, for 15 minutes or so before slicing.
Meanwhile, set the pot over medium-high heat and bring the onion mixture to a simmer. Add the spinach and tomatoes and stir until the spinach has wilted and the tomatoes have heated through. Slice the chicken, removing all the meat. Toss the meat with the spinach and tomato mixture.