Heat half of the oil in a heavy deep skillet over medium high heat. Saute the onion until tender, about 5 minutes, and then stir in the tomatoes, spinach and capers. Cook over medium low heat until everything is thick and tender.
While the veggies are cooking, put the rest of the oil in another large skillet (if you have a cast iron one, this is the time to break it out!), sprinkle the tuna with salt and pepper and sear the tuna for 3-5 minutes on each side, depending on how rare you like it.
Slice the tuna into strips and serve covered with the tomato spinach sauce. It’s delectable all on its own, but if you want some grains with your tuna, serve over polenta or rice.