Wash potatoes and cut into 1/4" to 1/2" cubes.
Cut onions into 1/4" to 1/2" cubes.
Finely dice jalapenos.
Divide potatoes, onions and jalapenos into two equal piles.
Mix each pile of potato/onion/jalapeno mixture with 1/2 tsp. each of rosemary and salt and pepper to taste.
Tear off an 18" x 24" piece of aluminum foil and spray the center generously with cooking spray.
Place one pile of potato/onion/jalapeno mix on each piece of aluminum foil and fold into tent packs.
Cook directly on coals for five minutes.
Remove from direct heat on the coals to a grill grate near the coals. Cook for 40 more minutes, gently shaking the foil packs every fifteen minutes until done.