Step 1 Roast the chile over a burner flame or under the broiler until it is evenly charred on all sides. Cool, then slip off the skin and remove the stem and seeds. Cut the flesh into a fine dice.
Step 2 Blanch the tomatoes in boiling water for 30 seconds, then peel and seed. Chop the flesh into a medium dice.
Step 3Stack the sage leaves and slice crosswise into very thin strips.
Step 4Melt half the butter in a large skillet over medium heat. Season the fillets with 1 teaspoon salt and one-fourth teaspoon pepper, or to taste. Dredge in flour to coat lightly, shaking off the excess. Lay into the pan top side down and saute for 2 minutes, then carefully turn over and saute until done, 1 to 2 minutes longer depending on the thickness; a sharp knife inserted into the thickest part should slide in easily. Transfer to a platter and keep warm.
Step 5Pour the butter out of the pan and wipe it clean with a paper towel. Raise the heat under the pan, add the remaining butter and, as it starts to melt, add the sage. Saute, stirring constantly for about a minute, until the sage becomes fragrant and almost starts to crisp. Add the tomatoes and poblano and saute until they are warmed through. Season the sauce with one-fourth teaspoon salt and one-eighth teaspoon pepper, or to taste, and spoon over the fish.