Grilled Flank Steak with Red Onion Marmalade

This is from Culinary Thymes, a great cooking magazine. Not sure if it's still in publication.

Marinade:
2/3  cup  soy sauce
2/3  cup  vegetable oil
4  cloves  garlic, minced
2  Tbs  dark brown sugar
2  Tbs  tomato paste
2  Tbs  chili powder (use less if using a hot chili powder)
1  Tbs  ground cumin
1/2  tsp  cayenne pepper
2  flank steaks (about 1 to 1-1/2 lbs each)

Whisk marinade ingredients together. Place steaks in an extra-large plastic bag (or 2 large ones). Pour marinade into the bag(s). Marinate in the refrigerator for several hours, or overnight, flipping the bag several times.

Remove steaks and pat dry with a paper towel. Grill over med-hot fire about 4 - 5 minutes per side, or until meat is well-seared and dark brown. Let rest for 10 minutes or so, and cut into diagonal slices, across the grain. Serve with Red Onion Marmalade spooned on top.

Red Onion Marmalade:
1/4  cup  olive oil
1  Tbs  unsalted butter
3  red onions, thinly sliced
1/2  tsp  freshly ground black pepper
1  Tbs  sherry vinegar

Heat oil and butter in a large skillet, over low heat. Add onions and pepper. Cook for 30 - 40 minutes, or until the onions are very soft and sweet. Add sherry vinegar; stir and cook another 15 minutes.