Preheat the oven to 450 degrees. Line a baking sheet with aluminum foil.
In a large, nonstick skillet, heat 1 tablespoon olive oil over medium-high heat. Saute the onions for 5-6 minutes or until beginning to soften. Add the sugar, and saute another 2-3 minutes, until beginning to brown slightly. Add the white wine, and let simmer until the wine evaporates. Turn down the heat to low, add the Balsamic vinegar, and continue to cook, stirring occasionally, for another 5 minutes. (Total cooking time for the onions is about 15 minutes.) Remove from the heat; season to taste with salt and pepper.
Drizzle the tuna steaks with the remaining tablespoon of olive oil, rubbing it in on both sides, and season with salt and pepper.
Place the pistachio nuts in a shallow dish. Dip the tuna steaks into the pistachio nuts, pressing gently to create a crust on both sides of the tuna. Lay the crusted steaks gently on the baking sheet; season with additional salt, and a drizzle of olive oil.
Roast for 5-6 minutes for medium-rare. (Cook longer if you prefer it to be more cooked in the middle.) Alternately, you can sear the tuna for 2 minutes per side and just finish in the oven for a few minutes.
Place a mound of onions in the center of each plate and top with a tuna steak. Drizzle with Balsamic glaze.
*You can buy this in specialty shops, or you can make your own, by slowly reducing 1/2 cup of Balsamic vinegar down to 1 tablespoon on the stovetop. It will be syrupy, thick, and sweet.