Sprinkle both sides of tuna with salt and pepper.
Heat oil in nonstick skillet over medium-high heat. Add tuna; cook until browned on both sides and just slightly pink in center, about 8 minutes. Transfer tuna to platter.
To same skillet, add chicken broth and lemon juice. Heat to boiling. Remove from heat. Swirl in mustard and chives. Spoon sauce over tuna. Serve immediately. Makes 4 servings.