How to Make It
Heat the oil in a medium skillet over medium-high heat. Add the shallots, rosemary, ½ teaspoon salt, and ¼ teaspoon pepper. Cook, stirring occasionally, until tender, 2 to 3 minutes.
Add the wine to the skillet and cook over medium-high heat until syrupy, 15 to 20 minutes. Add the broth and cook until reduced to 3/4 cup, 15 to 20 minutes more.
Meanwhile, in a small bowl, mash together the butter and flour with a fork. Reduce heat to medium and add the butter mixture to the skillet. Cook, stirring, until thickened, about 1 minute. Strain through a fine-mesh sieve.