1. Preheat a grill to medium-high (if using a charcoal grill, coals are ready when you can hold your hand 5 inches above grill for just 3 to 4 seconds). Alternatively, heat a grill pan over medium-high heat until hot.

2. Season steaks with the salt and pepper. Grill, turning once, 7 minutes total for medium-rare. Transfer steaks to a cutting board, and let rest 5 minutes.

3. Meanwhile, grill tomato slices, turning once, until soft, about 4 minutes total.

4. Heat oil and butter in a large skillet over medium-high heat until hot but not smoking. Add onions; cook, stirring occasionally, until golden, about 8 minutes.

5. Cut steaks against the grain into 1/4-inch-thick slices. Spread each of the 4 baguette bottoms with 2 1/4 teaspoons hot mustard. Top with steak slices, tomatoes, onions, and the 4 baguette tops. Wrap sandwiches in waxed paper; refrigerate until ready to serve.