If using fennel, cut off and discard upper stalks of fennel bulbs, reserving some of the leafy fronds. Snip 2 tablespoons of the fronds for use in the marinade. Remove any wilted outer layers from bulbs; cut off a thin slice from base of each bulb. Wash and cut each fennel bulb or red onion lengthwise into 6 wedges. Cook wedges, covered, in a small amount of lightly salted boiling water about 5 minutes or until nearly tender; drain. Place fennel or onion wedges in a resealable plastic bag set in a shallow bowl.
Trim fat from pork. Cut pork into 1-1/2-inch cubes. Place pork cubes in bag with fennel or onion. Set aside.
In a small bowl, stir together, snipped fennel fronds, if using, olive oil, lemon juice, oregano, fennel seeds, garlic, and salt. Pour over pork and fennel or onion wedges; seal bag. Marinate in the refrigerator for 1 to 2 hours, turning bag occasionally.
Drain pork and fennel or onion wedges, discarding marinade. On four 12- to 14-inch metal skewers, alternately thread pork cubes, fennel or onion wedges, and tomatoes, leaving a 1/4-inch space between pieces.
For a charcoal grill, grill skewers on the rack of an uncovered grill directly over medium coals for 18 to 20 minutes or until pork is no longer pink and juices run clear (160 degrees F), turning once halfway through grilling. (For a gas grill, preheat grill. Reduce heat to medium. Place skewers on grill rack over heat. Cover; grill as above.) Makes 4 servings.