Place fennel in 1-quart heatproof glass jar. Bring vinegar, sugar, peppercorns, 1 tsp salt, and ½ cup water to a boil in a medium saucepan, whisking to dissolve sugar and salt. Pour liquid into jar, cover, and set aside to cool to room temperature. Refrigerate until ready to use.

Place steak and 1 cup orange juice in a resealable plastic bag, pressing out as much air as possible before sealing. Refrigerate 6–8 hours. Let steak come to room temperature, about 1 hour. Prepare grill for medium-high heat and lightly oil grates.

Remove steaks from marinade and pat dry. Rub both side of steaks with vegetable oil; season with salt and pepper. Grill 5–7 minutes per side for medium-rare. Let rest 10 minutes before slicing across the grain.
Meanwhile, whisk lemon juice, orange juice, and olive oil in a medium bowl. Drain fennel and add to dressing along with onion, basil, mint, and chile, and toss to combine; season with salt and pepper.

Serve steak on top of fennel salad.