How to Make It
In a baking dish, whisk 1/2 cup of the olive oil with the wine, mustard, thyme and fennel. Arrange the tuna steaks in the dish, turn to coat, cover with plastic wrap and refrigerate for 15 minutes.
In a large cast-iron skillet, heat the remaining 1 tablespoon of olive oil. Remove the tuna from the marinade, letting the excess drip off, then season with salt and pepper. Cook the tuna over moderately high heat, turning once, until lightly browned but still pink in the center, about 3 minutes per side. Transfer the tuna to a work surface and let rest for 5 minutes.
Meanwhile, add the plums to the skillet and cook over moderate heat until tender but not falling apart, about 4 minutes. Add the maple syrup and lemon juice and cook until syrupy, about 1 minute.
Slice the tuna and transfer to plates. Spoon the plums and sauce over the tuna, top with Parmigiano-Reggiano cheese shavings and serve.