1. Preheat the oven to 220 degrees. Oil a 1 1/2 to 2 L gratin or baking dish. Remove the sardines from the oil and separate them into fillets. Set the oil aside.
2. Wilt the spinach either by steaming or blanching. To blanch bring a large pot of water to a boil, salt generously and add the spinach. Blanch for no more than 20 seconds (do this in batches). Transfer to a bowl of cold water, drain and squeeze out excess water. If you prefer, you can wilt the spinach by steaming for about 1 minute over an inch of boiling water. Chop medium-fine.
3. Heat the olive oil (not the oil from the sardines) in the skillet over medium heat and add the onion and a pinch of salt. Cook, stirring, until tender, 5 to 8 minutes. Add a generous pinch of salt, stir in the garlic and thyme and cook, stirring, until fragrant, 30 seconds to a minute, then add the chopped wilted spinach, flour, and salt and pepper to taste. Stir together for 1 minute, until everything is blended. Add the milk and cooked rice and stir together for about 1 minute, until you no longer see liquid in the pan. Remove from the heat. Taste and adjust salt and pepper.
4. Spread half the rice and spinach in the bottom of the baking dish. Top with the sardine fillets in one layer. Drizzle a tablespoon of the oil from the cans over the sardines, then top with the remaining rice and spinach in an even layer. Sprinkle on the breadcrumbs and drizzle on another tablespoon of the oil from the sardine cans. Place in the oven and bake 15 minutes, until sizzling. Serve hot or warm.