1. Preheat the oven to 400 degrees F. Combine the mustard and oil with a fork. Add the maple syrup, vinegar, 1/2 teaspoon salt, and 1/2 teaspoon pepper and stir briskly until combined.
2. Salt the flank steak on both sides and spoon half the marinade over the steak. Allow to marinate at room temperature, about 15 minutes.
3. In the meantime, wash the endive, pat dry, and cut into quarters, keeping the stems intact. Lay the endive on a baking sheet and drizzle with olive oil and salt to taste. Roast until the endive begins to soften and brown at the edges, about 15 minutes. Sprinkle with the lemon juice and garnish with parsley.
4. Turn the oven up to broil. Place the flank steak in the broiler and broil for 4 minutes. Flip and broil until nicely charred and an instant read thermometer reads 110 degrees F, about 4 minutes on the other side. Let rest 10 minutes before slicing.
5. Slice the steak across the grain and spoon the remaining marinade over top. Serve with the endive and more mustard on the side.
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