Season the thighs with the salt and pepper.
In a wide, heavy-bottomed pot that has a lid, melt the butter over medium heat. Add the thighs, skin side down, and let them cook without moving them around for 7 minutes. You are encouraging good caramelization of the skin and developing a ton of flavor in the process. After 7 minutes turn the thighs over and add the onions, paprika, and garlic to the pot. Cook for 5 minutes and then add the vinegar, being careful not to let it flame up.
This is a good time to get a spatula and loosen up all of those pan drippings. The vinegar needs to cook down by half, and when it does, add the stock. Cover and reduce the heat to low. Cook for 20 minutes over low heat, remove the lid, and add the oranges, mint, and spinach. Stir lightly and serve immediately.