Rinse the fish and dry the surface and the cavity of each thoroughly with paper towels. In a large nonstick frying pan, heat 2 tablespoons of the oil with 1 tablespoon of the butter over moderately high heat.
Sprinkle the trout with 1/2 teaspoon salt and 1/4 teaspoon pepper.
Put the trout in the pan and cook until golden, about 5 minutes. Turn and cook until golden on the second side and just done, 5 to 10 minutes longer.
Remove; wipe out the pan.
In the same pan, heat the remaining 1 tablespoon oil over moderately high heat.
Add the sage leaves and cook until crisp, about 1 minute. Alternatively, add the dried sage and cook, stirring, for 1 minute.
Pour the oil and sage over the fish. Wipe out the pan.
Add the wine to the pan. Boil until reduced to approximately 3 tablespoons, 1 to 2 minutes. Reduce the heat to the lowest setting and whisk in the remaining 4 tablespoons butter,1/8 teaspoon salt, and 1/8 teaspoon pepper.
Whisk in any accumulated juices and oil from the trout.
Pour the sauce around the fish and serve.
Test-Kitchen Tip: For a creamy emulsified butter sauce, don't just put the butter in the pan and let it melt; instead, stick a whisk in the cold butter and stir it around the bottom of the pan over the lowest possible heat. This way, the butter just softens into a sauce instead of melting into a puddle.
Wine Recommendation: Farm-raised trout are mild, so a pinot grigio or pinot bianco from northeastern Italy will make a superb accompaniment. A gamier wild fish can use a stronger wine such as a chardonnay.
