Heat the remaining 1 teaspoon oil in the pan over medium-low heat. Add grapes and cook, stirring occasionally and pressing down on them with the back of a spoon, until the grapes are brown in spots and mostly broken down, 4 to 6 minutes. Add shallot and cook, stirring, until fragrant, about 1 minute. Sprinkle with flour and stir to coat. Add port, broth, thyme and the remaining 1/4 teaspoon salt. Increase the heat to medium-high and cook, stirring and scraping up any browned bits, until reduced and thickened, 2 to 3 minutes. Serve the steaks with about 3 tablespoons sauce each.