Score bison flank steak on both sides by making shallow cuts at 1-inch intervals diagonally across the steak in a diamond pattern. Place bison flank steak between two pieces of plastic wrap. Using the flat side of a meat mallet, pound bison flank steak lightly into a 8x10 inch rectangle about ½ inch thick. Discard plastic wrap. Place bison flank steak in a resealable plastic bag set in a shallow bowl. For marinade, combine the 3 tablespoons oil, the lemon juice, the 2 cloves minced garlic, and the oregano in a medium bowl. Pour marinade over bison flank steak. Seal bag; turn to coat bison flank steak. Marinate in the refrigerator for 2 to 4 hours, turning bag occasionally

For filling, heat the 2 teaspoons oil over medium heat in a large skillet. Cook the 2 cloves garlic in hot oil for 1 minute. Add spinach; cook and stir for 1 to 2 minutes or until spinach begins to wilt. Drain spinach in a colander, pressing out any excess liquid with the back of a spoon. Coarsely chop spinach. Stir in feta cheese and kalamata olives; set aside.

Drain bison flank steak; discard marinade. Spread filling mixture evenly over bison flank steak to within 1 inch of edges. Starting from long edge, roll up bison flank steak into a spiral, making sure the ends stay tucked in. Using heavy 100-percent-cotton kitchen string, tie bison roll at 1-inch intervals. Place bison roll on a rack in a shallow roasting pan. Insert an oven-going meat thermometer into the center of the bison roll.

roast, uncovered, for 35 to 40 minutes or until internal temperature registers 135 degrees F. Cover with foil; let stand for 10 minutes before slicing. Slice bison roll into ¾ inch-thick slices.

To Grill: For charcoal grill, arrange preheated coals around a drip pan. Test for medium heat above pan. Insert a meat thermometer into the center of the bison roll. Place bison roll on grill rack over drip pan. Cover and grill for 35 to 40 minutes or until thermometer registers 135 degrees F. (For gas grill, preheat grill. Reduce heat to medium-high. Adjust for indirect grilling. Grill as above.) Cover with foil; let stand for 10 minutes before slicing.