In a 4- to 6-quart slow cooker, combine the broth, onion, garlic, Worcestershire, and flour. Season the brisket with ½ teaspoon salt and ¼ teaspoon pepper and place in the slow cooker. Cover and cook until very tender, on low for 7 to 8 hours or on high for 4 to 5 hours (this will shorten total recipe time).
Ten minutes before serving, transfer the brisket to a cutting board and thinly slice against the grain. Strain the cooking liquid into a small saucepan and reserve the onions. Boil the cooking liquid until reduced to 1½ cups, 5 to 7 minutes.
In a large bowl, whisk together the oil, vinegar, and ¼ teaspoon each salt and pepper. Add the greens and toss to combine.
Dividing evenly, form sandwiches with the baguette, brisket, and reserved onions. Serve with the cooking liquid, for dipping, and the salad.