Halve leek lengthwise, and slice into 1/4 inch thick pieces. Add oil to same skillet used to cook chicken, add leek and cook, stirring often, until softened and browned around edges, 2-3 minutes. Add flour and cook, stirring constantly for 30 seconds. Add broth and vermouth. Increase heat to medium-high, and bring to simmer, scraping pan bottom to loosen browned bits. Simmer rapidly for 3 - 5 minutes until reduced to 3/4 cup, Stir in any accumulated chicken juices, return to simmer and cook for 30 seconds. Turn off heat, whisk in mustard, tarragon and butter, season with salt and pepper, Spoon over cutlets and serve immediately.