Place fennel seeds and peppercorns in a small zip-top plastic bag. Seal bag and pound spices using a mallet or rolling pin until coarsely ground, or pulse them briefly in a spice or coffee grinder. Sprinkle mixture and 1/4 teaspoon salt on both sides of chops.

Heat 2 tablespoons oil in a large, high-sided skillet over medium heat. Add sweet potatoes; cook, stirring occasionally, until fork-tender, 10 to 12 minutes. Sprinkle with 1/4 teaspoon salt, then transfer to a plate.

Heat 1 tablespoon oil in the same skillet over medium-high heat. Add chops and reduce heat to medium; cook, turning once, about 4 minutes, or until an instant-read thermometer inserted into thickest parts registers 145 degrees F (63 degrees C). Transfer chops to a plate and tent with foil.

Add remaining 1 tablespoon oil and the onion to the skillet; cook until onion softens, about 2 minutes. Add apple; cook 1 minute more. Add broth, stirring to scrape up browned bits, then stir in mustard and sweet potatoes. Increase heat to medium-high; simmer until apple is slightly softened and liquid has slightly reduced, 1 to 2 minutes.

Add kale and remaining 1/4 teaspoon salt; cook, stirring occasionally, until kale is tender, about 1 minute. Add any accumulated juices from the plate with pork. Serve chops with vegetable mixture.