Combine all ingredients in medium bowl. Reserve 1/2 cup for basting. With a sharp knife, score both sides of steak in a crisscross pattern. Place steak in marinade and refrigerate for an hour. Grill (either at home or trailside) 15 to 20 minutes for medium-rare steak, baste occasionally with marinade, turning once. Carve diagonally across the grain into thin slices. Cut baguette on diagonal into 1/2-inch slices. Assemble mini-sandwiches with steak slices and bread.
If you can’t bring a grill to the picnic, grill the steaks at home and warm in a pan over the fire—or eat cold.