How to Make It
In a small saucepan, melt the butter. Add the shallots and cook over moderate heat, stirring occasionally, until just starting to soften and brown, about 3 minutes. Add the wine and bring to a boil, then simmer over moderately high heat until reduced to 1/2 cup, about 5 minutes. Add the fish stock and simmer over moderately low heat until the liquid is reduced to 1/2 cup, about 10 minutes; discard the shallots.
In a food processor, puree the sun-dried tomatoes with the garlic until nearly smooth. Add the olives and pulse until a coarse paste forms. Add the 2 tablespoons of olive oil and pulse to incorporate. Scrape the mixture into a medium bowl and whisk in the wine reduction. Season the sauce with salt.
Preheat a large cast-iron skillet or griddle over high heat. Brush the tuna steaks with olive oil and season generously with salt. Cook over high heat, turning once, until lightly browned outside and rare within, about 2 minutes total. Transfer to plates and serve with the sauce.