Sprinkle both sides yellowfin tuna steaks with kosher salt and freshly ground pepper.
Heat a heavy skillet over high heat. Add olive oil and 1 tablespoon of the butter, and swirl to coat the pan. Sear tuna steaks until golden brown on each side, turning only once. Do not overcook tuna. Remove and keep warm.
Lower heat to medium. Add the remaining tablespoon of butter to the pan along with the onions. Gently saute until onions are translucent. Add garlic and saute 1 additional minute, stirring often. Carefully pour in the white wine and lemon juice. Stir, and cook until the liquid is reduced by half, about 2 to 3 minutes. Add heavy cream, return to a simmer. Cook another 3 minutes or so until thickened. Stir in drained capers.
Pour lemon cream sauce over warm tuna steaks, and garnish with dill and lemon or lime to serve.